Build log-level plugin on its stable-3.1 branch

Change-Id: Ifd45e58e15190eea88f5cc9b4a2d675eebefa43c
diff --git a/jenkins/gerrit-bazel-plugin-log-level.yaml b/jenkins/gerrit-bazel-plugin-log-level.yaml
index ef14f1a..06f596a 100644
--- a/jenkins/gerrit-bazel-plugin-log-level.yaml
+++ b/jenkins/gerrit-bazel-plugin-log-level.yaml
@@ -4,5 +4,6 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-3.0
             - stable-2.16