[media] cec.h/cec-funcs.h: add option to use BSD license

Like the videodev2.h and other headers, explicitly allow these headers
to be used with the BSD license.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>