Add Nico and Scott to OWNERS for ninja related things

Bug:
Change-Id: Ib88e822f074242a362869a8636cb4d96c838a51b
Reviewed-on: https://chromium-review.googlesource.com/650232
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>

OWNERS

@@ -14,3 +14,5 @@ tandrii@chromium.org
 per-file commit_queue*=phajdan.jr@chromium.org
 per-file commit_queue*=sergiyb@chromium.org
 per-file commit_queue*=tandrii@chromium.org
+per-file ninja*=thakis@chromium.org
+per-file ninja*=scottmg@chromium.org