output: pass the music chunk pointer as void*, not char*

The meaning of the chunk depends on the audio format; don't suggest a
specific format by declaring the pointer as "char*", pass "void*"
instead.

src/output_control.h

@@ -50,7 +50,7 @@ void
 audio_output_signal(struct audio_output *ao);
 
 void
-audio_output_play(struct audio_output *ao, const char *chunk, size_t size);
+audio_output_play(struct audio_output *ao, const void *chunk, size_t size);
 
 void audio_output_pause(struct audio_output *ao);