avformat/tee: allow packets with negative timestamps

As this is a meta muxer and the same flag is set with the fifo
meta muxer, there is really no reason not to have this set here
as well.

Signed-off-by: Jan Ekström <jan.ekstrom@24i.com>

libavformat/tee.c

@@ -614,5 +614,5 @@ AVOutputFormat ff_tee_muxer = {
     .write_trailer     = tee_write_trailer,
     .write_packet      = tee_write_packet,
     .priv_class        = &tee_muxer_class,
-    .flags             = AVFMT_NOFILE | AVFMT_ALLOW_FLUSH,
+    .flags             = AVFMT_NOFILE | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
 };