Fix unreferenced errors with mingw compiler and unit tests

tests/basic/keymap.c

@@ -40,4 +40,7 @@ const macro_t *action_get_macro(keyrecord_t *record, uint8_t id, uint8_t opt) {
         }
     }
     return MACRO_NONE;
-};
+};
+
+void action_function(keyrecord_t *record, uint8_t id, uint8_t opt) {
+}