vorbiscomment: add DISCNUMBER to the metadata conv table

patch by Anton Khirnov  anton _at_ khirnov _dot_ net

Originally committed as revision 25357 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/vorbiscomment.c

@@ -32,6 +32,7 @@
 const AVMetadataConv ff_vorbiscomment_metadata_conv[] = {
     { "ALBUMARTIST", "album_artist"},
     { "TRACKNUMBER", "track"  },
+    { "DISCNUMBER",  "disc"   },
     { 0 }
 };