network: Link with libxml2

Since fb9f6ce6 we are including a libxml header file in the
network driver but never link with it. This hasn't caused an
immediate problem because in the end the network driver links
with libvirt.la. But apparently, it's causing a build issue on
old Ubuntu.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/network/Makefile.inc.am

@@ -47,7 +47,7 @@ libvirt_driver_network_impl_la_CFLAGS = \
 	$(AM_CFLAGS) \
 	$(NULL)
 libvirt_driver_network_impl_la_SOURCES = $(NETWORK_DRIVER_SOURCES)
-libvirt_driver_network_impl_la_LIBADD  = $(DBUS_LIBS)
+libvirt_driver_network_impl_la_LIBADD  = $(DBUS_LIBS) $(LIBXML_LIBS)
 
 libexec_PROGRAMS += libvirt_leaseshelper
 libvirt_leaseshelper_SOURCES = $(NETWORK_LEASES_HELPER_SOURCES)