Make p-devel's selinux-policy-devel dep conditional on selinux-policy
    
    The policycoreutils-devel package is used not only for working with an
    installed policy, but also for building the policy from sources. In the
    latter case, there is no need to install selinux-policy-devel (and
    selinux-policy along with it), so make the dependency conditional on
    selinux-policy.
    
    Since policy is often built from source in a mock chroot or a container,
    this will avoid the awkward and unnecessary cyclic build dependency of
    selinux-policy on itself.
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>