setup.py: make executable

It's common form to be able to execute setup.py directly.

BUG=None
TEST=`./setup.py ...` works

Change-Id: I3155eca1016cc87d2c64562c15aff19cbb058860
Reviewed-on: https://chromium-review.googlesource.com/716341
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>