gstreamer1.0-libav: upgrade to version 1.18.0

License-Update: All code is now LGPL-2.1+

Starting with 1.17, we will not ship a copy of FFmpeg in our release
tarballs, and all the remaining code is LGPL2.
https://gitlab.freedesktop.org/gstreamer/gst-libav/-/commit/c437e175ad9c82732ac46e36ccc398ad041bf13b



Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>