staging: vboxvideo: select DRM_TTM

vboxvideo code uses various ttm_* functions, so it needs
"drivers/gpu/drm/ttm/*" to be built. In order to build these objects
select DRM_TTM.

This commit fixes "ttm_*" undefined build errors.

Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>