Add rnk@ to tools/clang/OWNERS

This should give us better Pacific time coverage for update.py and
package.py changes.

R=thakis@chromium.org,hans@chromium.org

Change-Id: I713033f33f755fce6b4c8e5aa4387cf2d4b3a245
Reviewed-on: https://chromium-review.googlesource.com/1154043Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578953}

tools/clang/OWNERS

 hans@chromium.org
 pcc@chromium.org
 thakis@chromium.org
+rnk@chromium.org
 
 # Only for clang tooling.
 dcheng@chromium.org