metainfo: Add launchable
Flathub switched to appstreamcli, hence we need to pass its validation in order to publish new builds.
This commit fixes the appstreamcli error:
$ flatpak run --command=flatpak-builder-lint org.flatpak.Builder appstream *metainfo*
E: org.gnome.Firmware.desktop:~: desktop-app-launchable-omitted
This `desktop-application` component has no `desktop-id` launchable tag, however it contains all
the necessary information to display the application. The omission of the launchable entry means
that this application can not be launched directly from installers or software centers. If this
is intended, this information can be ignored, otherwise it is strongly recommended to add a
launchable tag as well.
Note that .desktop in the app id is not needed.