Mark as Multi-Arch: foreign, like x-d-p-gtk
This makes it possible to satisfy the Depends/Recommends of both flatpak:amd64 and steam:i386 simultaneously.
xdg-desktop-portal users such as like Flatpak apps and Steam/Proton games communicate with the portal (and indirectly its backends) via an architecture-independent D-Bus API, so it does not matter whether the caller, the portal and the backends are all the same architecture.
Closes: #985914
Not tested, but the change is fairly obvious. Please cherry-pick to experimental, too.