Build websession-flatfile on stable-3.3

Change-Id: I38c9872e3c5e5bae2aece3bb5c81d26fe26f3fe2
diff --git a/jenkins/gerrit-bazel-plugin-websession-flatfile.yaml b/jenkins/gerrit-bazel-plugin-websession-flatfile.yaml
index 53c4639..dd60391 100644
--- a/jenkins/gerrit-bazel-plugin-websession-flatfile.yaml
+++ b/jenkins/gerrit-bazel-plugin-websession-flatfile.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.3
             - stable-3.2
             - stable-3.1
             - stable-3.0