Add GstLibav using the default system ffmpeg
Follow up for https://gitlab.gnome.org/GNOME/gnome-sdk-images/merge_requests/3
- Targeting gnome-3-28
- Building gst-libav 1.12.4 as other gnome modules in that branch
Follow up for https://gitlab.gnome.org/GNOME/gnome-sdk-images/merge_requests/3