build: -avoid-version on libvirt_driver_nwfilter
    
    This fixes the following make rpm warning:
    
    warning: Installed (but unpackaged) file(s) found:
       /usr/lib64/libvirt/connection-driver/libvirt_driver_nwfilter.so.0
       /usr/lib64/libvirt/connection-driver/libvirt_driver_nwfilter.so.0.0.0
    
    introduced in comit 8d559864
    
    Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
    Signed-off-by: Eric Blake <eblake@redhat.com>