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.c

@@ -45,6 +45,11 @@ void decoder_initialized(struct decoder * decoder,
 	notify_signal(&pc.notify);
 }
 
+const char *decoder_get_url(mpd_unused struct decoder * decoder, char * buffer)
+{
+	return get_song_url(buffer, dc.current_song);
+}
+
 enum decoder_command decoder_get_command(mpd_unused struct decoder * decoder)
 {
 	return dc.command;

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);
 
 /**

src/inputPlugins/wavpack_plugin.c

@@ -456,7 +456,7 @@ static int wavpack_streamdecode(struct decoder * decoder, InputStream *is)
 		 * As we use dc.utf8url, this function will be bad for
 		 * single files. utf8url is not absolute file path :/
 		 */
-		utf8url = get_song_url(tmp, dc.current_song);
+		utf8url = decoder_get_url(decoder, tmp);
 		if (utf8url == NULL) {
 			break;
 		}