Add serviceuser plugin to the check pipeline

Change-Id: Ifc74ba451b3b44ff0f07abc01a8a18e118dce398

zuul.d/projects.yaml

diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index 30e9e6f..d6b66ec 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -205,3 +205,9 @@
     check:
       jobs:
         - gerrit-plugin-build
+
+- project:
+    name: plugins/serviceuser
+    check:
+      jobs:
+        - gerrit-plugin-build