Fix lets_split.c so that audio can be disabled

Matching the use of `#ifdef AUDIO_ENABLE` used in `matrix_init_kb()` in order to compile firmware for the Let's Split keyboard without audio enabled.

keyboards/lets_split/lets_split.c

@@ -24,7 +24,9 @@ void matrix_init_kb(void) {
 };
 
 void shutdown_user(void) {
+    #ifdef AUDIO_ENABLE
         PLAY_NOTE_ARRAY(tone_goodbye, false, 0);
 	_delay_ms(150);
 	stop_all_notes();
+    #endif
 }