Add martiniss@ to //testing/buildbot/OWNERS.

Change-Id: I4185507af93c77d0e2514769dfbd66376591302f
Reviewed-on: https://chromium-review.googlesource.com/1042791Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555811}

testing/buildbot/OWNERS

@@ -7,6 +7,7 @@ jbudorick@chromium.org
 jochen@chromium.org
 kbr@chromium.org
 machenbach@chromium.org
+martiniss@chromium.org
 maruel@chromium.org
 sky@chromium.org
 tansell@chromium.org