Change include guards in quantum/ to pragma once (#11239)

quantum/process_keycode/process_audio.h

@@ -1,5 +1,4 @@
-#ifndef PROCESS_AUDIO_H
-#define PROCESS_AUDIO_H
+#pragma once
 
 float compute_freq_for_midi_note(uint8_t note);
 
@@ -9,5 +8,3 @@ void process_audio_noteoff(uint8_t note);
 void process_audio_all_notes_off(void);
 
 void audio_on_user(void);
-
-#endif