post_build_ninja_summary.py: chmod u+x

Makes it easier to run on linux/mac.

Change-Id: I0ebf512cd576a5c8aafbddc7c6b6f8e2bd6919d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3971922
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>