Add stable-2.15 to avatars-external

Change-Id: I204506915567e1ba2763374ebb45793065ca7cba
diff --git a/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml b/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml
index c5abd1b..55c8a1e 100644
--- a/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml
@@ -9,4 +9,5 @@
             - master
           gerrit-branch:
             - master
+            - stable-2.15
             - stable-2.14