fullscreen-shell: Add missing license tag

Although it would probably default to the license at the root of the
repository anyway, it's best to be explicit about it, and also be
consistent with the other extensions.

The copyright holders have been assembled from git history and the
README.

Signed-off-by: Johan Klokkhammer Helsing <johan.helsing@qt.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>