Add `Recommends: (rpm-plugin-selinux if selinux-policy)`
    
    This pairs with https://src.fedoraproject.org/rpms/selinux-policy/pull-request/370
    
    Basically I want to break the hard selinux-policy -> rpm dependency,
    but this should help ensure that we don't somehow lose rpm-plugin-selinux
    on package-based systems.
    
    Only people disabling recommends would be at some risk.  We could
    also choose to upgrade this to a `Requires`.