Add gavinmak@ to owners

Change-Id: I309bfc112966a84c55d881fe874abfc7842c4a6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2698787
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>

OWNERS

@@ -5,6 +5,7 @@ dpranke@google.com
 ehmaldonado@chromium.org
 ehmaldonado@google.com
 jochen@chromium.org
+gavinmak@google.com
 sokcevic@google.com
 
 per-file ninja*=thakis@chromium.org