appdata: Fix metainfo license
Note that this merge request is for the gnome-3-28 branch.
appstream-generator (as used by at least Debian and Ubuntu) was emitting metainfo-license-invalid which prevented the Appstream metadata from being used on those distros.
This was already fixed in master with commit bc86062a
Closes: https://gitlab.gnome.org/GNOME/gnome-games/issues/77