Build reject-private-submit against gerrit stable-3.6

Bug: Issue 15831
Change-Id: Ie35f5193ac2726f3263d0b9883cf290fd08f9fae
diff --git a/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml b/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml
index 32cd320..e48ae5b 100644
--- a/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml
+++ b/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3