Exclude tools/lintstack.head.py for pep8 check

Commit 062ac3313d7fd39611a6b065ffdff4bf309d8b17 has enabled pep8
check on tools directory, however, if developers have run
`tox -epylint` before, then `tox -epep8` will fail because the
automatically generated file tools/lintstack.head.py cannot pass
pep8 check.

This patch excludes tools/lintstack.head.py in tox pep8 env.

Change-Id: Idbf22821f3d44055f8a03321aa0a05416bec809f