Need to pull AwaitInputChange function into the C interface

[craiglindley]

I have been struggling for 2 days trying to make this work and I am still not successful. I am using adafruit-hat-pwm. In led-matrix-c.h I added this line:

/** CAL Await GPIO pin change */
int led_matrix_await_input_change(struct RGBLedMatrix *matrix);

In led-matrix-c.cc I added these lines:

int led_matrix_await_input_change(struct RGBLedMatrix *matrix) {
return to_matrix(matrix)->AwaitInputChange(100);
}

Everything compiles and links OK but I never see activity on GPIO 19 which was said to be free by your input-example program.