Build the project-group-structure plugin on stable-3.0

Change-Id: Ie991e173d95872835d72bfa08ec46489352c510d
diff --git a/jenkins/gerrit-buck-bazel-plugin-project-group-structure.yaml b/jenkins/gerrit-buck-bazel-plugin-project-group-structure.yaml
index 16a6203..8eb8d2b 100644
--- a/jenkins/gerrit-buck-bazel-plugin-project-group-structure.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-project-group-structure.yaml
@@ -7,6 +7,7 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.0
             - stable-2.16
             - stable-2.15
             - stable-2.14