qt5webengine: use QT5WEBENGINE prefix for CHROMIUM_LICENSE_FILES variable

The qt5webengine package currently uses the CHROMIUM_LICENSE_FILES
variable to hold the list of license files for the chromium source
code embedded inside the qt5webengine code.

However, using this variable would clash with a hypothetical
"chromium" package, and anyway violates our rule that all variables of
a package should be prefixed by the package name.

This commit fixes that by adding the QT5WEBENGINE to this variable.

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