lavf/vc1test: add rcv to vc1test demuxer extensions

rcv is commonly used as extension for vc1 test stream files.
Signed-off-by: Jun Zhao <jun.zhao@intel.com>

libavformat/vc1test.c

@@ -122,5 +122,6 @@ AVInputFormat ff_vc1t_demuxer = {
     .read_probe     = vc1t_probe,
     .read_header    = vc1t_read_header,
     .read_packet    = vc1t_read_packet,
+    .extensions     = "rcv",
     .flags          = AVFMT_GENERIC_INDEX,
 };