spdifenc: set flag AVFMT_NOTIMESTAMPS

There are no timestamps in IEC 61937.
Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit 6c60fcf8)

libavformat/spdifenc.c

@@ -545,5 +545,6 @@ AVOutputFormat ff_spdif_muxer = {
     spdif_write_header,
     spdif_write_packet,
     spdif_write_trailer,
+    .flags = AVFMT_NOTIMESTAMPS,
     .priv_class = &class,
 };