Build x-docs plugin on stable-2.15

Change-Id: I754ab4e538699e42fec907e34e9f0f9170b6f9a1
diff --git a/jenkins/gerrit-buck-bazel-plugin-x-docs.yaml b/jenkins/gerrit-buck-bazel-plugin-x-docs.yaml
index 6dad719..aba8662 100644
--- a/jenkins/gerrit-buck-bazel-plugin-x-docs.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-x-docs.yaml
@@ -9,4 +9,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-2.15
             - stable-2.14