sandbox: sandbox_serial_pending depends on DM_VIDEO

sandbox: sandbox_serial_pending depends on DM_VIDEO

When building sandbox_defconfig with CONFIG_DM_VIDEO=n a link time error
occurs:

   in function `sandbox_serial_pending':
   drivers/serial/sandbox.c:101: undefined reference to `video_sync_all'

video_sync_all() is only defined if we have CONFIG_DM_VIDEO=y.

Calling this function in a serial driver looks quite hackish
but at least let's add the missing build constraint.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>