[buster] install failure : libav-tools missing

[arthurlutz]

Testing ampache_ynh on a buster install (cf https://github.com/YunoHost/issues/issues/1255 )

I can't install ampache and get the following : https://paste.yunohost.org/raw/culecamugi

[snip]
2020-02-24 15:09:10,281: DEBUG - dpkg: ampache-ynh-deps: dependency problems, but configuring anyway as you requested:
2020-02-24 15:09:10,281: DEBUG -  ampache-ynh-deps depends on libav-tools; however:
2020-02-24 15:09:10,281: DEBUG -   Package libav-tools is not installed.
[snip]

libav-tools was a transition packages to ffmpeg if I understand correctly.

cc @alexAubin who is working on the buster branches for yunohost.