Fix AppStream duplicate key error

When trying to upgrade the Flatpak to GNOME Runtime 46, I was getting the following error:

{
    "errors": [
        "appstream-failed-validation"
    ],
    "appstream": [
        "E: org.gnome.TwentyFortyEight:27: custom-key-duplicated Purism::form_factor"
    ],
    "message": "Please consult the documentation at https://docs.flathub.org/docs/for-app-authors/linter"
}

Which can be seen in the build run here: https://buildbot.flathub.org/#/builders/19/builds/15370

I replaced it with the control tag, which is detailed here:

https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-relations-control

Finally I upstreamed the patch here: https://github.com/flathub/org.gnome.TwentyFortyEight/blob/master/gnome-2048-meson.patch