build: require netcf-0.2.2 when installing on Fedora18+

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.