Cut the memory consumption of PREVENT_STUCK_MODIFIERS in half

tmk_core/common/action_layer.h

@@ -71,6 +71,9 @@ void layer_xor(uint32_t state);
 #endif
 
 
+/* return the topmost non-transparent layer currently associated with key */
+int8_t layer_switch_get_layer(keypos_t key);
+
 /* return action depending on current layer status */
 action_t layer_switch_get_action(keypos_t key);