[media] videodev2.h: add __user to v4l2_ext_control pointers

These are not copied to kernel space by video_usercopy, so mark them
as __user.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>