media: dev-encoder.rst: add GFDL-1-1+ to SPDX header

The media documents under the uAPI should be GFDL compatible.

So, make this one dual-licensed GPL-2.0 or GFDL-1.1+ with
no-invariant sections.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>