added decoder_get_url()

The wavpack decoder plugin implements a hack, and it needs the song
URL for that.  This API (and the hack) should be revised later, but
add that function for now.

src/decoder_api.h

@@ -103,6 +103,8 @@ void decoder_initialized(struct decoder * decoder,
 			 const AudioFormat * audio_format,
 			 float total_time);
 
+const char *decoder_get_url(struct decoder * decoder, char * buffer);
+
 enum decoder_command decoder_get_command(struct decoder * decoder);
 
 /**