Makefile: Fix build without driver modules

Makefile: Fix build without driver modules

After 87dea4fcf 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: Michal Privoznik <mprivozn@redhat.com>