MKV: force packetization of MPEG audio frames

Fixes #11479
The file has some MP3 frames, but the frames are not packetized correctly in the Matroska file (spec violation)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/matroska_segment_parse.cpp

@@ -1494,6 +1494,7 @@ int32_t matroska_segment_c::TrackInit( mkv_track_t * p_tk )
              !strcmp( p_tk->psz_codec, "A_MPEG/L1" ) )
     {
         p_tk->fmt.i_codec = VLC_CODEC_MPGA;
+        p_fmt->b_packetized = false;
     }
     else if( !strcmp( p_tk->psz_codec, "A_AC3" ) )
     {