Add GstLibav using the default system ffmpeg
Follow up for https://gitlab.gnome.org/GNOME/gnome-sdk-images/merge_requests/3
- Targeting the "master" branch of the Sdk
- Building gst-libav master branch
Note that building was not tested as I got many failure (first because of several servers not supporting shallow clones which is what is done by flatpak-buider 0.10.6) and after working around that I am getting other issues (not related to gst-libav itself).