Build librest explicitly for Flatpak
Since librest has been removed from the runtime we need to build it (again) in the manifest.
Long-term we probably need to migrate away from librest, as it's not maintained. And going forward the needed parts might instead be integrated directly into gnome-online-accounts (and ported to libsoup 3). We might need to do something similar.
But for the time being, re-add it be built in the Flatpak manifest.