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>
Merge pull request #14 from abelal/add_lmbench_license

common-licenses: Add license for lmbench.