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/decoder_api.c

@@ -18,6 +18,7 @@
  */
 
 #include "decoder_api.h"
+#include "audio.h"
 
 #include "utils.h"
 #include "normalize.h"