Use Requires(meta): (rpm-plugin-selinux if rpm-libs)

We support two ways to update the operating system:

- `/usr/bin/rpm` (and `dnf` etc.) where SELinux labels are
  computed and written client side
- ostree (and other image-based systems) where SELinux labels
  were computed server side.

In the ostree case, I'd like the ability to generate smaller
images that do not even have `rpm` installed.

This hard dependency from `selinux-policy` -> `rpm` is one of
the only main blockers.

RPM supports these "alternative" conditionals, it's easy to do.

Related: RHEL-54303

selinux-policy.spec

@@ -69,7 +69,7 @@ BuildRequires: make
 BuildRequires: systemd-rpm-macros
 Requires(pre): policycoreutils >= %{POLICYCOREUTILSVER}
 Requires(post): /bin/awk /usr/bin/sha512sum
-Requires(meta): rpm-plugin-selinux
+Requires(meta): (rpm-plugin-selinux if rpm-libs)
 Requires: selinux-policy-any = %{version}-%{release}
 Provides: selinux-policy-base = %{version}-%{release}
 Suggests: selinux-policy-targeted