Build multi-site against gerrit stable-3.6

Bug: Issue 15831
Change-Id: I20c4e965ec71d5a94d8dbaa15ffffecc63f310b0
diff --git a/jenkins/gerrit-bazel-plugin-multi-site.yaml b/jenkins/gerrit-bazel-plugin-multi-site.yaml
index 1325471..742b36b 100644
--- a/jenkins/gerrit-bazel-plugin-multi-site.yaml
+++ b/jenkins/gerrit-bazel-plugin-multi-site.yaml
@@ -5,6 +5,7 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.6
             - stable-3.5
       - 'plugin-{name}-bazel-{branch}':
           branch: