Install gssproxy mechanism config in mech.d
    
    This requires at least Fedora version 1.11.5 where mech.d support was
    backported or upstreams > 1.12 version.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Guenther Deschner <gdeschner@redhat.com>