Build its-base plugin on stable-3.6

Bug: Issue 15831

Change-Id: Id8afc6ca18001d8ecc6fe08331660af3460e1ad9
diff --git a/jenkins/gerrit-bazel-plugin-its-base.yaml b/jenkins/gerrit-bazel-plugin-its-base.yaml
index d0294a4..9706793 100644
--- a/jenkins/gerrit-bazel-plugin-its-base.yaml
+++ b/jenkins/gerrit-bazel-plugin-its-base.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.6
             - stable-3.5
             - stable-3.4
             - stable-3.3