common-licenses: Add license for lmbench.

lmbench has a distribution restriction which is covered through adding some
constraints on the GPLv2 license. Adding that specific license here. This license
is available under the lmbench source tree as COPYING-2.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
(cherry picked from commit 22c5c2dd6e758856aeb1009cad520f102da31271)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>