Makefile: Fix build without driver modules

After 87dea4fc one can observe a build failure:

./autogen.sh --system --without-driver-modules && make

  CCLD     libvirtd
../src/.libs/libvirt_driver_vbox.a(libvirt_driver_vbox_impl_la-vbox_driver.o):
In function `vboxNetworkRegister':
/home/jtomko/work/libvirt/libvirt.git/src/vbox/vbox_driver.c:168: undefined
reference to `vboxGetNetworkDriver'
collect2: error: ld returned 1 exit status
make[3]: *** [libvirtd] Error 1

The problem is that when building without driver modules the VBOX
network driver is not linked into the the VBOX driver.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/Makefile.am

@@ -1171,6 +1171,7 @@ noinst_LTLIBRARIES += libvirt_driver_vbox.la
 # GPLv2-only license requries that it be linked into
 # libvirtd and *not* libvirt.so
 #libvirt_la_BUILT_LIBADD += libvirt_driver_vbox.la
+libvirt_driver_vbox_la_LIBADD += libvirt_driver_vbox_network_impl.la
 endif ! WITH_DRIVER_MODULES
 
 libvirt_driver_vbox_impl_la_CFLAGS =				\