decoder_plugin: added method container_scan()

[mk: fixed whitespace errors; use delete_song() instead of
songvec_delete()]

src/decoder/sidplay_plugin.cxx

@@ -156,6 +156,7 @@ const struct decoder_plugin sidplay_decoder_plugin = {
 	NULL, /* stream_decode() */
 	sidplay_file_decode,
 	sidplay_tag_dup,
+	NULL, /* container_scan */
 	sidplay_suffixes,
 	NULL, /* mime_types */
 };