cdg: enable seeking

Signed-off-by: NMichael Niedermayer <michaelni@gmx.at>

libavformat/cdg.c

@@ -65,5 +65,6 @@ AVInputFormat ff_cdg_demuxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("CD Graphics Format"),
     .read_header    = read_header,
     .read_packet    = read_packet,
+    .flags= AVFMT_GENERIC_INDEX,
     .extensions = "cdg"
 };