ffmpeg: fix license file

Somewhere between 1.2.7 and 2.3.1 (version bump in bf76f431)
the LICENSE file has been renamed to LICENSE.md, so `make legal-info`
is currently broken. Update to the current file name.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>