spec file: Add Requires(post) on selinux-policy
    
    This prevents ipa-server-upgrade failures on SELinux AVCs because of old
    selinux-policy version.
    
    https://fedorahosted.org/freeipa/ticket/5256
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Martin Kosek <mkosek@redhat.com>