libvirt.spec.in: Use libnl-devel for RHEL-6
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 3694e038fd12825042cca6204f7aa7322e2ed9c9
  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

jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,eskultety/libvirt,taget/libvirt,olafhering/libvirt,zippy2/libvirt,fabianfreyer/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jardasgit/libvirt,taget/libvirt,jardasgit/libvirt,datto/libvirt,jardasgit/libvirt,nertpinx/libvirt,datto/libvirt,olafhering/libvirt,eskultety/libvirt,jfehlig/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,andreabolognani/libvirt,datto/libvirt,olafhering/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,datto/libvirt,fabianfreyer/libvirt,eskultety/libvirt,taget/libvirt,zippy2/libvirt,nertpinx/libvirt,taget/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,taget/libvirt,jardasgit/libvirt,crobinso/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,libvirt/libvirt,VenkatDatta/libvirt,zippy2/libvirt,andreabolognani/libvirt,datto/libvirt,libvirt/libvirt,eskultety/libvirt,eskultety/libvirt