Add dstockwell to third_party/ink OWNERS

Change-Id: I0b9d04f69d5dba38bdcd0889648023d3ad5939cf
Reviewed-on: https://chromium-review.googlesource.com/c/1394353Reviewed-by: François Degros <fdegros@chromium.org>
Commit-Queue: dstockwell <dstockwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619792}

third_party/ink/OWNERS

+dstockwell@chromium.org
 dvallet@chromium.org
 fdegros@chromium.org
 martiw@chromium.org