Fix RPM spec for wireshark on Fedora < 24
    
    This previous commit
    
      commit cd9fcc8be7dcb9126d70f744ce54b0b742eeefb8
      Author: Michal Privoznik <mprivozn@redhat.com>
      Date:   Wed Jul 27 16:58:32 2016 +0200
    
        libvirt.spec.in: Adapt to newest wireshark plugindir
    
    Adapted the libvirt spec for wireshark >= 2.1.0 but
    this ignored the fact that we enable wireshark from
    Fedora 21 and 2.1.0 was only added in Fedora 24
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>