output/shout: declare metadata as UTF-8

Apparently, Icecast defaults to ISO-8859-1 for MP3:

 http://icecast.org/docs/icecast-2.4.0/config-file.html#mountsettings

This change forces Icecast to UTF-8 without having to configure it in
Icecast's configuration file.

src/output/plugins/ShoutOutputPlugin.cxx

@@ -383,6 +383,7 @@ ShoutOutput::SendTag(const Tag &tag)
 		shout_tag_to_metadata(tag, song, sizeof(song));
 
 		shout_metadata_add(meta, "song", song);
+		shout_metadata_add(meta, "charset", "UTF-8");
 		if (SHOUTERR_SUCCESS != shout_set_metadata(shout_conn, meta)) {
 			LogWarning(shout_output_domain,
 				   "error setting shout metadata");