decoder_api: moved struct decoder_plugin to decoder_plugin.h

The decoder_plugin struct is used by both the MPD core and the decoder
plugin implementations.  Move it to a shared header file, to minimize
header dependencies.

src/decoder_list.c

@@ -17,12 +17,14 @@
  */
 
 #include "decoder_list.h"
-#include "decoder_api.h"
+#include "decoder_plugin.h"
 #include "utils.h"
 #include "config.h"
 
 #include <glib.h>
 
+#include <string.h>
+
 extern const struct decoder_plugin mp3Plugin;
 extern const struct decoder_plugin vorbis_decoder_plugin;
 extern const struct decoder_plugin flac_decoder_plugin;