py/stackctrl: Prevent unused-var warning when stack checking disabled.

Signed-off-by: Damien George <damien@micropython.org>

py/stackctrl.h

@@ -40,7 +40,7 @@ void mp_stack_check(void);
 
 #else
 
-#define mp_stack_set_limit(limit)
+#define mp_stack_set_limit(limit) (void)(limit)
 #define MP_STACK_CHECK()
 
 #endif