examples: Try harder to uninstall nwfilter

We have this code in our Makefile that tries to remove
/etc/libvirt/nwfilter if directory is left empty after all our
example nwfilters were uninstalled. However, the check for that
is missing quotation marks thus rendering the test useless:

test -z allow-arp.xml allow-dhcp-server.xml .. qemu-announce-self.xml || \
  rmdir "/some/path/libvirt.git/_install/etc/libvirt/nwfilter"
/bin/sh: line 0: test: too many arguments
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

examples/Makefile.am

@@ -90,5 +90,5 @@ uninstall-local::
 	for f in $(FILTERS); do \
 		rm -f "$(NWFILTER_DIR)/`basename $$f`"; \
 	done
-	-test -z $(shell ls $(NWFILTER_DIR)) || rmdir $(NWFILTER_DIR)
+	-test -z "$(shell ls $(NWFILTER_DIR))" || rmdir $(NWFILTER_DIR)
 endif WITH_NWFILTER