Add tikuta to OWNERS under build directory

I did more than 50 commits under build directory, and I'd like to have ownership of build configs especially for goma and build performance related things.

~/chromium/src$ git log --author=tikuta --oneline build | wc -l
52

Change-Id: I3c0549ab4fc1426a37e5d6c0d7253c264da3ebc3
Reviewed-on: https://chromium-review.googlesource.com/1152707
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579013}

build/OWNERS

@@ -5,6 +5,7 @@ jochen@chromium.org
 scottmg@chromium.org
 thakis@chromium.org
 brucedawson@chromium.org
+tikuta@chromium.org
 
 # Clang build config changes
 hans@chromium.org