lintian: Stop exporting LINTIAN_LAB and remove @MUST_EXPORT

Stop exporting LINTIAN_LAB as nothing relies on it anymore.  Since
that leaves only LINTIAN_ROOT in @MUST_EXPORT, remove that as well
(and export LINTIAN_ROOT manually).

Signed-off-by: Niels Thykier <niels@thykier.net>