drv_pwm_output: include board config header

- board config can define PX4_PWM_ALTERNATE_RANGES which needs to be known
by the compiler prior to processing this file

Signed-off-by: Roman <bapstroman@gmail.com>

src/drivers/drv_pwm_output.h

@@ -46,6 +46,7 @@
 
 #include <stdint.h>
 #include <sys/ioctl.h>
+#include <board_config.h>
 
 #include "drv_orb_dev.h"