output_plugin: the plugin allocates the audio_output object

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

src/output_api.h

@@ -21,6 +21,7 @@
 #define MPD_OUTPUT_API_H
 
 #include "output_plugin.h"
+#include "output_internal.h"
 #include "audio_format.h"
 #include "tag.h"
 #include "conf.h"