build: require netcf-0.2.2 when installing on Fedora18+

A previous patch forced libnl-3 and netcf-0.2.2 (which itself requires
libnl-3) when *building* for Fedora 18+ (and RHEL 7+), but the
install-time Requires: for netcf has always been implicit due to
libvirtd linking with libnetcf.so. However, the since the API of netcf
didn't change when it was rebuilt to use libnl-3, the internal library
version didn't change either, making it possible (from rpm's point of
view) to upgrade libvirt without upgrading netcf (in reality, that
leads to a segfault - see
https://bugzilla.redhat.com/show_bug.cgi?id=853381).

The solution is to put an explicit Requires: line in libvirt's
specfile for fedora >= 18 and rhel >= 7.

libvirt.spec.in

@@ -589,6 +589,9 @@ Requires: iptables-ipv6
 %if %{with_nwfilter}
 Requires: ebtables
 %endif
+%if %{with_netcf} && (0%{?fedora} >= 18 || 0%{?rhel} >= 7)
+Requires: netcf-libs >= 0.2.2
+%endif
 # needed for device enumeration
 %if %{with_hal}
 Requires: hal