libvirt.spec.in: Use libnl-devel for RHEL-6

RHEL-6 still needs to use libnl instead of libnl3, so re-add
the spec conditional mistakenly removed in

  commit 3694e038
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Wed May 4 15:43:08 2016 +0100

    libvirt.spec.in: drop Fedora < 20 and RHEL < 6

libvirt.spec.in

@@ -302,7 +302,11 @@ BuildRequires: yajl-devel
 BuildRequires: sanlock-devel >= 2.4
 %endif
 BuildRequires: libpcap-devel
+%if 0%{?rhel} && 0%{?rhel} < 7
+BuildRequires: libnl-devel
+%else
 BuildRequires: libnl3-devel
+%endif
 BuildRequires: avahi-devel
 BuildRequires: libselinux-devel
 BuildRequires: dnsmasq >= 2.41