Build serviceuser plugin for 2.15

Change-Id: I246af4c12dbf03210573c7ed47a0f0a0d7152d9c
diff --git a/jenkins/gerrit-buck-bazel-plugin-serviceuser.yaml b/jenkins/gerrit-buck-bazel-plugin-serviceuser.yaml
index 65f0694..57d63d2 100644
--- a/jenkins/gerrit-buck-bazel-plugin-serviceuser.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-serviceuser.yaml
@@ -10,4 +10,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-2.15
             - stable-2.14