set default subtitle_codec for ass muxer

Originally committed as revision 25870 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/assenc.c

@@ -83,6 +83,7 @@ AVOutputFormat ass_muxer = {
     .mime_type      = "text/x-ssa",
     .extensions     = "ass,ssa",
     .priv_data_size = sizeof(ASSContext),
+    .subtitle_codec = CODEC_ID_SSA,
     .write_header   = write_header,
     .write_packet   = write_packet,
     .write_trailer  = write_trailer,