mpegts: Mark the muxer as supporting variable fps

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/mpegtsenc.c

@@ -1280,6 +1280,6 @@ AVOutputFormat ff_mpegts_muxer = {
     .write_header   = mpegts_write_header,
     .write_packet   = mpegts_write_packet,
     .write_trailer  = mpegts_write_end,
-    .flags          = AVFMT_ALLOW_FLUSH,
+    .flags          = AVFMT_ALLOW_FLUSH | AVFMT_VARIABLE_FPS,
     .priv_class     = &mpegts_muxer_class,
 };