nrf5: Exclude import of pwm.h in modmachine.c if MICROPY_PY_MACHINE_PWM is not set, as nrf51 does not yet have this module yet.
nrf5: Exclude import of pwm.h in modmachine.c if MICROPY_PY_MACHINE_PWM is not set, as nrf51 does not yet have this module yet.

tralamazza/micropython,tralamazza/micropython,adafruit/micropython,tralamazza/micropython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,tralamazza/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython