stm32: Consolidate include of genhdr/pins.h to single location in pin.h.

genhdr/pins.h is an internal header file that defines all of the pin
objects and it's cleaner to have pin.h include it (where the struct's for
these objects are defined) rather than an explicit include by every user.

ports/stm32/pin.h

@@ -63,6 +63,9 @@ typedef struct {
 extern const mp_obj_type_t pin_type;
 extern const mp_obj_type_t pin_af_type;
 
+// Include all of the individual pin objects
+#include "genhdr/pins.h"
+
 typedef struct {
   const char *name;
   const pin_obj_t *pin;