Add ref-protection plugin to the checks pipeline

Change-Id: I8e30f592a09a58d9d1a2aff61efe6ece446fb89d

zuul.d/projects.yaml

diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index 5e8ae95..72aef60 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -255,6 +255,12 @@
         - gerrit-plugin-build
 
 - project:
+    name: plugins/ref-protection
+    check:
+      jobs:
+        - gerrit-plugin-build
+
+- project:
     name: plugins/rename-project
     check:
       jobs: