output: added output_api.h

Just like decoder_api.h, output_api.h provides the audio output API
which is used by the plugins.

src/audioOutput.c

@@ -21,6 +21,7 @@
 #include "list.h"
 #include "log.h"
 #include "pcm_utils.h"
+#include "utils.h"
 #include "os_compat.h"
 #include "audio.h"