Set default subtitle_codec to CODEC_ID_TEXT

Originally committed as revision 10355 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskaenc.c

@@ -740,6 +740,7 @@ AVOutputFormat matroska_muxer = {
     mkv_write_packet,
     mkv_write_trailer,
     .codec_tag = (const AVCodecTag*[]){codec_bmp_tags, codec_wav_tags, 0},
+    .subtitle_codec = CODEC_ID_TEXT,
 };
 
 AVOutputFormat matroska_audio_muxer = {