leds: qcom-lpg: add missing PWM dependency

The Qualcomm LPG driver fails to probe unless PWM support is enabled so
add the missing Kconfig dependency.

Fixes: 24e2d05d ("leds: Add driver for Qualcomm LPG")
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>