build: only install nwfilter examples when building nwfilter

build: only install nwfilter examples when building nwfilter

'make rpm' with client_local set to 1 (by manual modification,
or with RHEL 5 on s390) warns:

RPM build errors:
    Installed (but unpackaged) file(s) found:
   /etc/libvirt/nwfilter/allow-arp.xml
   /etc/libvirt/nwfilter/allow-dhcp-server.xml
   /etc/libvirt/nwfilter/allow-dhcp.xml
   /etc/libvirt/nwfilter/allow-incoming-ipv4.xml
   /etc/libvirt/nwfilter/allow-ipv4.xml
   /etc/libvirt/nwfilter/clean-traffic.xml
   /etc/libvirt/nwfilter/no-arp-ip-spoofing.xml
   /etc/libvirt/nwfilter/no-arp-mac-spoofing.xml
   /etc/libvirt/nwfilter/no-arp-spoofing.xml
   /etc/libvirt/nwfilter/no-ip-multicast.xml
   /etc/libvirt/nwfilter/no-ip-spoofing.xml
   /etc/libvirt/nwfilter/no-mac-broadcast.xml
   /etc/libvirt/nwfilter/no-mac-spoofing.xml
   /etc/libvirt/nwfilter/no-other-l2-traffic.xml
   /etc/libvirt/nwfilter/no-other-rarp-traffic.xml
   /etc/libvirt/nwfilter/qemu-announce-self-rarp.xml
   /etc/libvirt/nwfilter/qemu-announce-self.xml
   /usr/share/polkit-1/actions/org.libvirt.api.policy

The bulk of these are fixed with this patch.

* examples/xml/nwfilter/Makefile.am (install-data-local)
(uninstall-local): Make conditional.

Signed-off-by: Eric Blake <eblake@redhat.com>