Build reject-private-submit plugin on stable-3.4 branch

Change-Id: I693464542835b912644f32b3dec2d0b3b0e11d8b
diff --git a/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml b/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml
index a980385..2bba48e 100644
--- a/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml
+++ b/jenkins/gerrit-bazel-plugin-reject-private-submit.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.4
             - stable-3.3
       - 'plugin-{name}-bazel-{branch}':
           branch: