audio_format: converted typedef AudioFormat to struct audio_format

Get rid of CamelCase, and don't use a typedef, so we can
forward-declare it, and unclutter the include dependencies.

src/timer.h

@@ -19,16 +19,17 @@
 #ifndef MPD_TIMER_H
 #define MPD_TIMER_H
 
-#include "audio_format.h"
 #include "os_compat.h"
 
+struct audio_format;
+
 typedef struct _Timer {
 	uint64_t time;
 	int started;
 	int rate;
 } Timer;
 
-Timer *timer_new(AudioFormat *af);
+Timer *timer_new(struct audio_format *af);
 
 void timer_free(Timer *timer);