Build find-owners plugin on stable-2.15

Change-Id: I930713061158ad4c7cf9ccdffbd673f8da5ebf60
diff --git a/jenkins/gerrit-bazel-plugin-find-owners.yaml b/jenkins/gerrit-bazel-plugin-find-owners.yaml
index 5cecef9..68147f2 100644
--- a/jenkins/gerrit-bazel-plugin-find-owners.yaml
+++ b/jenkins/gerrit-bazel-plugin-find-owners.yaml
@@ -4,4 +4,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-2.15
             - stable-2.14