ffmpeg7 on netbsd-9

[nia <nia%NetBSD.org@localhost>]

ffmpeg7 has been failing on netbsd-9 for a while, specifically in
the v4l (videoio/webcam) code. I assume that netbsd-10 has a newer
<sys/videoio.h> so it isn't a problem there.

ffmpeg has a custom configure system, so it's not like autotools
where it would be easy to disable the v4l detection.

I'm not sure of the best solution here -
copy in a more recent v4l header?