Add brucedawson@ to own post_build_ninja_summary.py

Bruce created the script and is most qualified to review changes to it.

Bug: None
Change-Id: I238c88ad03db422f9c7f5b2760ff2dd25d431e65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2141224
Commit-Queue: Peter Wen <wnwen@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

OWNERS

@@ -11,6 +11,7 @@ per-file ninja*=thakis@chromium.org
 per-file ninja*=scottmg@chromium.org
 
 per-file autoninja*=brucedawson@chromium.org
+per-file post_build_ninja_summary.py=brucedawson@chromium.org
 
 per-file ninjalog*=tikuta@chromium.org