avformat/movenc: allow negative TS for the ipod muxer

Fixes Ticket2708
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/movenc.c

@@ -4070,7 +4070,7 @@ AVOutputFormat ff_ipod_muxer = {
     .write_header      = mov_write_header,
     .write_packet      = mov_write_packet,
     .write_trailer     = mov_write_trailer,
-    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH,
+    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
     .codec_tag         = (const AVCodecTag* const []){ codec_ipod_tags, 0 },
     .priv_class        = &ipod_muxer_class,
 };