mxfdec: set AVFMT_SEEK_TO_PTS demuxer flag

Since 53f2ef2c seeking is done using PTS.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/mxfdec.c

@@ -3210,6 +3210,7 @@ static int mxf_read_seek(AVFormatContext *s, int stream_index, int64_t sample_ti
 AVInputFormat ff_mxf_demuxer = {
     .name           = "mxf",
     .long_name      = NULL_IF_CONFIG_SMALL("MXF (Material eXchange Format)"),
+    .flags          = AVFMT_SEEK_TO_PTS,
     .priv_data_size = sizeof(MXFContext),
     .read_probe     = mxf_probe,
     .read_header    = mxf_read_header,