Build menuextender plugin on stable-3.4 branch

Change-Id: I78cee162cac9a3da837351ee396c8c7b193200b8
diff --git a/jenkins/gerrit-bazel-plugin-menuextender.yaml b/jenkins/gerrit-bazel-plugin-menuextender.yaml
index c32bacc..35b7afe 100644
--- a/jenkins/gerrit-bazel-plugin-menuextender.yaml
+++ b/jenkins/gerrit-bazel-plugin-menuextender.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.4
             - stable-3.3
             - stable-3.2
             - stable-3.1