Claims:1. An apparatus for voltage regulation in a wireless power receiver, comprising: a rectifier comprising an output voltage; and voltage compensation logic to reduce voltage variation of the output voltage from the rectifier. 2. The apparatus of claim 1, wherein the voltage compensation logic comprises a voltage controlled capacitor in parallel with a wireless power receiving coil. 3. The apparatus of claim 1, wherein the voltage compensation logic generates a compensated current source configured to limit a power range of the rectifier within a predefined range. 4. The apparatus of claim 3, wherein limiting the power range is a function of voltage range during load modulation of the wireless power receiver. 5. The apparatus of claim 3, wherein the current source comprises: a first amplifier to generate a first voltage associated with a lower limit of the voltage range; a voltage divider to generate a second voltage associated with a higher limit of the voltage range; a second amplifier to: receive the first voltage and the second voltage; and supply an equalized output voltage within the predefined range based on the first voltage and the second voltage. 6. The apparatus of claim 1, wherein the voltage comparison logic comprises: a voltage controlled capacitor; a difference amplifier configured to: detect a difference between a reference voltage and a voltage associated with the output from the rectifier; and adjust a bias voltage output from the difference amplifier supplied to the voltage controlled capacitor based on the detected difference. 7. The apparatus of claim 6, wherein the bias voltage output from the difference amplifier is within a predefined continuous range. 8. The apparatus of claim 6, wherein the voltage controlled capacitor is a first voltage controlled capacitor, further comprising a second voltage controlled capacitor in series with the receiver coil, and wherein the first voltage controlled capacitor and the second voltage controlled capacitor are configured to supply a minimum operating voltage to the rectifier. 9. The apparatus of claim 1, wherein the voltage comparison logic comprises a difference amplifier configured to: detect a difference between a reference voltage and a voltage associated with output from the rectifier; and trigger an associated value in a capacitor bank to reduce capacitance of the voltage controlled capacitor. 10. The apparatus of claim 9, wherein a bias voltage output from the difference amplifier is within a predefined range of discrete voltages. 11. A method for voltage regulation in a wireless power receiver, comprising: detecting an output voltage of a rectifier; and reducing voltage variation of the output voltage via voltage compensation logic. 12. The method of claim 11, wherein reducing voltage variation of the output voltage via voltage comprises driving a voltage controlled capacitor in parallel with a wireless power receiving coil. 13. The method of claim 11, further comprising limiting a power range of the rectifier within a predefined range as a function of rectified voltage. 14. The method of claim 13, wherein the power range is limited during load modulation of the wireless power receiver. 15. The method of claim 13, wherein limiting the voltage range via the current source comprises: generating a first voltage associated with a lower limit of the voltage range at a first amplifier; generating a second voltage associated with a higher limit of the voltage range at a voltage divider; receiving the first voltage and the second voltage at a second amplifier; and providing, from the second amplifier, an equalized output voltage within the predefined range based on the first voltage and the second voltage. 16. The method of claim 11, wherein reducing voltage variation of the output voltage via voltage controlled capacitors comprises: detecting a difference between a reference voltage and a voltage associated with the output from the rectifier; and adjusting a bias voltage output from the difference amplifier to be provided to the voltage controlled capacitor based on the detected difference. 17. The method of claim 16, wherein the bias voltage output from the difference amplifier is within a predefined continuous range. 18. The method of claim 16, wherein the voltage controlled capacitor is a first voltage controlled capacitor among a second voltage capacitor, the second voltage capacitor in series with the receiver coil, the method further comprising configuring the first and second voltage capacitor to provide a minimum operating voltage to the rectifier. 19. The method of claim 1, wherein t reducing voltage variation of the output voltage via voltage controlled capacitors comprises: detecting a difference between a reference voltage and a voltage associated with output from the rectifier; and triggering an associated value in a capacitor bank to reduce capacitance of the voltage controlled capacitor. 20. The method of claim 19, wherein a bias voltage output from the difference amplifier is within a predefined range of discrete voltages. 21. A system for wireless charging, comprising: a rectifier comprising an output voltage; and voltage compensation logic to reduce voltage variation of the output voltage from the rectifier, wherein the voltage compensation logic comprises: a capacitor in parallel with a wireless power receiving coil; a current source configured to limit a voltage range of the rectifier within a predefined range; or any combination thereof. 22. The system of claim 21, wherein limiting the voltage range generates a limit in power range at the rectifier during load modulation of the wireless power receiver. 23. The system of claim 21, wherein the current source comprises: a first amplifier to generate a first voltage associated with a lower limit of the voltage range; a voltage divider to generate a second voltage associated with a higher limit of the voltage range; a second amplifier to: receive the first voltage and the second voltage; and provide an equalized output voltage within the predefined range based on the first voltage and the second voltage. 24. The system of claim 21, wherein the capacitor is a voltage controlled capacitor, and wherein the voltage comparison logic further comprises a difference amplifier configured to: detect a difference between a reference voltage and a voltage associated with the output from the rectifier; and adjust a bias voltage output from the difference amplifier to be provided to the voltage controlled capacitor based on the detected difference. 25. The system of claim 21, wherein the capacitor is a first voltage controlled capacitor, further comprising a second voltage controlled capacitor in series with the receiver coil, and wherein the first voltage controlled capacitor and the second voltage controlled capacitor are configured to provide a minimum operating voltage to the rectifier. , Description:Technical Field This disclosure relates generally to techniques for wireless charging. Specifically, this disclosure relates to regulation of voltage in wireless power systems. Background Art A basic wireless charging system may include a wireless power transmitter unit (PTU) and a wireless power receiving unit (PRU). For example, a PTU may include a transmit (Tx) coil, and a PRU may include receive (Rx) coil. Magnetic resonance wireless charging may employ a magnetic coupling between the Tx coil and the Rx coil. In some cases, wireless power received may vary causing issues with efficiency in wireless charging systems. In some cases, variations of voltage may violate wireless charging standard specifications. Brief Description of the Drawings Fig. 1 is block diagram of a PTU to provide power to a PRU, wherein the includes logic configured to reduce variations in voltage; Fig. 2 is an illustration of logic configured to reduce variations of rectifier voltage; Fig. 3 is a graph illustrating changes in voltage in terms of capacitance; Fig. 4 is a graph illustrating changes in rectifier voltage in terms of resistance in a magnetic coupling between a wireless transmitter and a wireless receiver; Fig. 5 is an illustration of logic configured to reduce variations of rectifier power using a compensated current source that may change as a function of rectified voltage; Fig. 6 is a graph illustrating changes in voltage at the rectifier in terms of current; Fig. 7 is a time line illustrating changes in voltage at the rectifier in terms of current enabled by a current source; and Fig. 8 is a flow diagram of a method for reducing voltage variation in a rectifier of wireless charging apparatus. The same numbers are used throughout the disclosure and the figures to reference like components and features. Numbers in the 100 series refer to features originally found in Fig. 1; numbers in the 200 series refer to features originally found in Fig. 2; and so on. Detailed Description The present disclosure relates generally to techniques for wireless charging. Specifically, the techniques described herein include a wireless power rectifier and voltage compensation logic to reduce voltage variation of output voltage from the rectifier. As discussed above, voltage variations may generate inefficiencies in wireless charging. For example, spatial freedom of placement of a device under charge having a receiver (Rx) coil may result in a potentially large rectified receiver voltage (Vrect) variation. Variations in Vrect may constrain designs by requiring stringent field uniformity in some cases. Voltage compensation logic, as referred to herein, includes one or more electric circuit components, modules, or integrated components configured to reduce variation in voltage. Specifically, voltage compensation logic may be configured to reduce variation of Vrect at a wireless charging component such as a wireless charging receiving coil. As discussed in more detail below, voltage compensation logic may include one or more of capacitors in a feedback loop configured to reduce variations in Vrect, amplifiers, resistors, and the like. In some cases, large variation on Vrect may present different design challenges, as discussed below in regard to Fig. 2 – Fig. 4. One design challenge may include delivering load modulation signals within a predefined range. For example, in some cases power (Prect) of load modulation may be required to be between .5 watts (W) and 1.1 W. Therefore, in some cases, the voltage compensation logic and techniques discussed herein may generate a compensated current source (I_compensate) that may be used to toggle load modulation signaling on Vrect (for any Vrect, for example Vrect = 4Volts-20Volts), wherein .5W