V4L/DVB (9898): v4l2: Add privacy control

The privacy control prevents video from being acquired by the camera. A true
value indicates that no image can be captured. Devices that implement the
privacy control must support read access and may support write access.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>