media: pixfmt-meta-d4xx.rst: Add a license to it

This file is a recent addition to media docs, and it is now the
only one without any license.

While the best is to dual-license with GFDL and GPL, it is,
at least, compatible with GFDL, as this is a requirement to be
part of the media uAPI docs.

So, add such license to it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/pixfmt-meta-d4xx.rst

-.. -*- coding: utf-8; mode: rst -*-
+.. Permission is granted to copy, distribute and/or modify this
+.. document under the terms of the GNU Free Documentation License,
+.. Version 1.1 or any later version published by the Free Software
+.. Foundation, with no Invariant Sections, no Front-Cover Texts
+.. and no Back-Cover Texts. A copy of the license is included at
+.. Documentation/media/uapi/fdl-appendix.rst.
+..
+.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
 
 .. _v4l2-meta-fmt-d4xx: