qt5webengine: add separate file for chromium license files for Qt LTS

The qtwebengine package used by Qt LTS has a different set of license
files for chromium, so this commit introduces a separate
chromium-lts.inc with the right contents.

This fixes legal-info of qt5webengine with Qt LTS.

Signed-off-by: Laurent Hartanerot <laurent.hartanerot@atos.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>