rtpenc: Set the AVFMT_TS_NONSTRICT flag

In particular, when packetizing mpegts into rtp, the input packet
timestamp may come from more than one stream, which could cause
multiple packets be written with the same timestamp.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc.c

@@ -632,4 +632,5 @@ AVOutputFormat ff_rtp_muxer = {
     .write_packet      = rtp_write_packet,
     .write_trailer     = rtp_write_trailer,
     .priv_class        = &rtp_muxer_class,
+    .flags             = AVFMT_TS_NONSTRICT,
 };