Add ffmpeg
To match 1.6 branch, see https://github.com/flatpak/freedesktop-sdk-images/commit/d693455104efc077f4cd6119a3ae8848ef19a793
Edited by Javier Jardón
To match 1.6 branch, see https://github.com/flatpak/freedesktop-sdk-images/commit/d693455104efc077f4cd6119a3ae8848ef19a793