Update OWNERS for //.gn to include //build/OWNERS

The top-level .gn file is used by the GN meta build system to
among other things set startup options. This is something that the
people working on our build system should be allowed to change
without having to involve top-level owners of the chromium
source tree, as they should have the necessary knowledge to
make good decisions.

BUG=None

Review-Url: https://codereview.chromium.org/2639593003
Cr-Commit-Position: refs/heads/master@{#444144}

OWNERS

@@ -8,6 +8,7 @@ per-file .gitattributes=*
 per-file .gitignore=*
 per-file .git-blame-ignore-revs=mgiuca@chromium.org
 per-file .git-blame-ignore-revs=thakis@chromium.org
+per-file .gn=file://build/OWNERS
 per-file AUTHORS=*
 per-file BUILD.gn=file://build/OWNERS
 per-file DEPS=*