Build SAML plugin for Gerrit v2.12

Change-Id: I9a4dddb39358cc2f361875758a8d38dedf185c09
diff --git a/jenkins/gerrit-sbt-plugin-saml.yaml b/jenkins/gerrit-sbt-plugin-saml.yaml
index 141038b..890300a 100644
--- a/jenkins/gerrit-sbt-plugin-saml.yaml
+++ b/jenkins/gerrit-sbt-plugin-saml.yaml
@@ -3,4 +3,5 @@
     jobs:
       - 'plugin-{name}-sbt-{branch}':
           branch:
+            - stable-2.12
             - stable-2.11