Filter out duplicates in the list of make file includes.
Filter out duplicates in the list of make file includes.

We can end up with duplicates if we are running gyp on two
files at the same time.

[ submitting for tony@chromium.org - http://codereview.chromium.org/1660014 ]
Review URL: http://codereview.chromium.org/1733003
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google