allow to trigger to the gitlab plugin using the jenkins pipeline model definition syntax

- [Pipeline model definition issue](https://issues.jenkins-ci.org/browse/JENKINS-40402)
 - [Pipeline model definition wiki](https://github.com/jenkinsci/pipeline-model-definition-plugin/wiki/Trigger-runs#other-available-triggers)

pom.xml

@@ -182,6 +182,11 @@
       <version>1.1</version>
       <optional>true</optional>
     </dependency>
+    <dependency>
+      <groupId>org.jenkins-ci</groupId>
+      <artifactId>symbol-annotation</artifactId>
+      <version>1.5</version>
+    </dependency>
     <dependency>
       <groupId>org.eclipse.jgit</groupId>
       <artifactId>org.eclipse.jgit</artifactId>

src/main/java/com/dabsquared/gitlabjenkins/GitLabPushTrigger.java

@@ -44,6 +44,7 @@ import jenkins.triggers.SCMTriggerItem.SCMTriggerItems;
 import net.karneim.pojobuilder.GeneratePojoBuilder;
 import net.sf.json.JSONObject;
 import org.apache.commons.lang.StringUtils;
+import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.Ancestor;
 import org.kohsuke.stapler.AncestorInPath;
 import org.kohsuke.stapler.DataBoundConstructor;
@@ -293,6 +294,7 @@ public class GitLabPushTrigger extends Trigger<Job<?, ?>> {
     }
 
     @Extension
+    @Symbol("gitlab")
     public static class DescriptorImpl extends TriggerDescriptor {
 
         private transient final SequentialExecutionQueue queue = new SequentialExecutionQueue(Jenkins.MasterComputer.threadPoolForRemoting);