libavformat: Update the comment about AVOutputFormat flags

Add a flag which applies here, which had been missed.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/avformat.h

@@ -458,7 +458,7 @@ typedef struct AVOutputFormat {
      * can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER,
      * AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,
      * AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS, AVFMT_ALLOW_FLUSH,
-     * AVFMT_TS_NONSTRICT
+     * AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE
      */
     int flags;