segment: drop global headers setting

avconv provides an option to set it externally and there isn't a way for
the outer demuxer to report the inner demuxer flag.

Solves a bad interaction between mpegts and x264, but requires
additional setting for the user.

libavformat/segment.c

@@ -319,7 +319,7 @@ AVOutputFormat ff_segment_muxer = {
     .name           = "segment",
     .long_name      = NULL_IF_CONFIG_SMALL("segment"),
     .priv_data_size = sizeof(SegmentContext),
-    .flags          = AVFMT_GLOBALHEADER | AVFMT_NOFILE,
+    .flags          = AVFMT_NOFILE,
     .write_header   = seg_write_header,
     .write_packet   = seg_write_packet,
     .write_trailer  = seg_write_trailer,