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>

drivers/staging/vboxvideo/Kconfig

@@ -2,6 +2,7 @@ config DRM_VBOXVIDEO
 	tristate "Virtual Box Graphics Card"
 	depends on DRM && X86 && PCI
 	select DRM_KMS_HELPER
+	select DRM_TTM
 	select GENERIC_ALLOCATOR
 	help
 	  This is a KMS driver for the virtual Graphics Card used in