Add symbol to OriginMergeRequestDiscoveryTrait descriptor (#119)

* Add symbol to OriginMergeRequestDiscoveryTrait descriptor

This give use an easier way to specify the trait in yaml template file

* Update OriginMergeRequestDiscoveryTrait.java

* Add missing import

Author: stevejulich

src/main/java/io/jenkins/plugins/gitlabbranchsource/OriginMergeRequestDiscoveryTrait.java

@@ -19,6 +19,7 @@
 import jenkins.scm.api.trait.SCMSourceTraitDescriptor;
 import jenkins.scm.impl.ChangeRequestSCMHeadCategory;
 import jenkins.scm.impl.trait.Discovery;
+import org.jenkinsci.Symbol;
 import org.kohsuke.accmod.Restricted;
 import org.kohsuke.accmod.restrictions.NoExternalUse;
 import org.kohsuke.stapler.DataBoundConstructor;
@@ -102,6 +103,10 @@ public boolean includeCategory(@NonNull SCMHeadCategory category) {
         return category instanceof ChangeRequestSCMHeadCategory;
     }
 
+    /**
+     * Our descriptor
+     */
+    @Symbol("gitLabOriginDiscovery")
     @Extension
     @Discovery
     public static class DescriptorImpl extends SCMSourceTraitDescriptor {