moved struct AudioFormat to audio_format.h

We want to expose the AudioFormat structure to plugins; remove some
clutter by moving its declaration to a separate header file.

src/timer.h

@@ -19,8 +19,8 @@
 #ifndef MPD_TIMER_H
 #define MPD_TIMER_H
 
-#include "audio.h"
-#include "mpd_types.h"
+#include "audio_format.h"
+#include "os_compat.h"
 
 typedef struct _Timer {
 	uint64_t time;