lavf/segment: unbreak behavior for segment muxer

The segment muxer presumes the use of AVFMT_GLOBALHEADER, ssegment should
be use in case of formats which requires no global headers.

Regression introduced when merging
0826d8513d.

libavformat/segment.c

@@ -561,7 +561,7 @@ AVOutputFormat ff_segment_muxer = {
     .name           = "segment",
     .long_name      = NULL_IF_CONFIG_SMALL("segment"),
     .priv_data_size = sizeof(SegmentContext),
-    .flags          = AVFMT_NOFILE,
+    .flags          = AVFMT_NOFILE|AVFMT_GLOBALHEADER,
     .write_header   = seg_write_header,
     .write_packet   = seg_write_packet,
     .write_trailer  = seg_write_trailer,