util: fix Linux build when libnl-devel isn't available

util: fix Linux build when libnl-devel isn't available

There was one stray bit of code in virnetdev.c that required libnl to
build, but wasn't qualified by defined(WITH_LIBNL). Adding that, plus
putting a similar check around a static function only used by that
aforementioned code, makes libvirt build properly without libnl3-devel
installed.

How useful it is in that state is a separate issue :-)

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>