Build websession-broker plugin on stable-3.13

Change-Id: Ie76e2bb68d1d4c84f95720b7c39d5ded431319e7
diff --git a/jenkins/gerrit-bazel-plugin-websession-broker.yaml b/jenkins/gerrit-bazel-plugin-websession-broker.yaml
index e9723de..6a12493 100644
--- a/jenkins/gerrit-bazel-plugin-websession-broker.yaml
+++ b/jenkins/gerrit-bazel-plugin-websession-broker.yaml
@@ -12,3 +12,4 @@
       - 'plugin-{name}-gh-bazel-{branch}':
           branch:
             - master
+            - stable-3.13