Build websession-flatfile plugin for v2.16

Change-Id: Idf2705e66b77a7ae372de49e15cecac739312568
diff --git a/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml b/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
index ca87cb6..c35ea23 100644
--- a/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
@@ -10,5 +10,6 @@
             - master
           gerrit-branch:
             - master
+            - stable-2.16
             - stable-2.15
             - stable-2.14