Add code-owners plugin to check pipeline

This adds the code-owners plugin to the check pipeline.  Alternatively,
this change could be made in the code owners repo itself in the
.zuul.yaml file.

Change-Id: I06175c01292f09c1b4a211d25a253e16568fd82c
Depends-On: https://gerrit-review.googlesource.com/267313
diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index c2f8184..f22fce4 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -71,3 +71,11 @@
     check:
       jobs:
         - gerrit-plugin-build
+
+# Other plugins:
+
+- project:
+    name: plugins/code-owners
+    check:
+      jobs:
+        - gerrit-plugin-build