update .gitignore
update .gitignore

Ignore build/ directory, which is auto-created when running `python
setup.py bdist` like dist/ directory.

Change-Id: Ie58936320d995e4e81f8929128475e538665bde8

openstack/swift-bench,jeblair/swift-bench,openstack/swift-bench,tobegit3hub/swift-bench,tobegit3hub/swift-bench