watchdog.c: fix compilation with module enabled.

There was a linker error regarding missing MASK function, which
is actually a macro defined in 'pinio.h'. After #including the
file, problem disappeared.

watchdog.c

@@ -15,6 +15,7 @@
 #include	<avr/wdt.h>
 #include	<avr/interrupt.h>
 #include	"memory_barrier.h"
+#include "pinio.h"
 
 #include	"arduino.h"
 #ifndef	EXTRUDER