Build websession-flatfile against stable-3.5

Bug: Issue 15285
Change-Id: I968cc6a29e424e9b35efcd1af6bbcf11f79124f5
diff --git a/jenkins/gerrit-bazel-plugin-websession-flatfile.yaml b/jenkins/gerrit-bazel-plugin-websession-flatfile.yaml
index ffabaf1..68443f0 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-3.5
             - stable-3.4
             - stable-3.3
             - stable-3.2