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_api.c

@@ -17,6 +17,7 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#include "decoder_api.h"
 #include "decoder_internal.h"
 #include "decoder_control.h"
 #include "player_control.h"