player_thread: log played song in "default" log mode

Very few lines to log a song URI when it has been entirely played.
Then mpd logs can be parsed to do statistics.

src/player_thread.c

@@ -567,8 +567,14 @@ play_next_chunk(struct player *player)
 static bool
 player_song_border(struct player *player)
 {
+	char *uri;
+
 	player->xfade = XFADE_UNKNOWN;
 
+	uri = song_get_uri(player->song);
+	g_message("played \"%s\"", uri);
+	g_free(uri);
+
 	music_pipe_free(player->pipe);
 	player->pipe = dc.pipe;