staging: vboxvideo: make a couple of symbols static

Module parameter vbox_modeset and structure vbox_bo_driver do not need to
be in global scope and hence can be made static.

Cleans up a couple of sparse warnings:
symbol 'vbox_modeset' was not declared. Should it be static?
symbol 'vbox_bo_driver' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Michael Thayer <michael.thayer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>