demux_lavf: v4l streams are not seekable

FFmpeg is retarded enough not to give us any indication whether it is
(unless we query fields not in the ABI/API). I bet FFmpeg developers
love it when library users have to litter their code with duplicated
information.

demux/demux_lavf.c

@@ -163,6 +163,8 @@ static const struct format_hack format_hacks[] = {
     {"mp4", .skipinfo = true, .fix_editlists = true},
     {"matroska", .skipinfo = true},
 
+    {"v4l2", .no_seek = true},
+
     // In theory, such streams might contain timestamps, but virtually none do.
     {"h264", .if_flags = AVFMT_NOTIMESTAMPS },
     {"hevc", .if_flags = AVFMT_NOTIMESTAMPS },