gifdec: rudimentary seeking

With this ffplay -loop X successfully seeks to start.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavformat/gifdec.c

@@ -286,5 +286,6 @@ AVInputFormat ff_gif_demuxer = {
     .read_probe     = gif_probe,
     .read_header    = gif_read_header,
     .read_packet    = gif_read_packet,
+    .flags          = AVFMT_GENERIC_INDEX,
     .priv_class     = &demuxer_class,
 };