fixed small bug with AUDIO_ENABLED (#339)

quantum/quantum.c

@@ -435,7 +435,7 @@ void matrix_scan_quantum() {
 
   matrix_scan_kb();
 }
-
+#ifdef AUDIO_ENABLE
   bool is_music_on(void) {
       return (music_activated != 0);
   }
@@ -458,5 +458,6 @@ void music_off(void) {
       stop_all_notes();
   }
 
+#endif
 __attribute__ ((weak))
 void music_on_user() {}