Add declarative Pipeline dependency to sample plugin

Plugin has a declarative Pipeline test in DeclarativePipelineTest.java

jenkinsci#2470 (comment)
provides more reasoning why the dependency should be included.

This reverts part of commit acad2e3.

sample-plugin/pom.xml

@@ -820,6 +820,11 @@
       <classifier>tests</classifier>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.jenkinsci.plugins</groupId>
+      <artifactId>pipeline-model-definition</artifactId>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.jenkinsci.plugins</groupId>
       <artifactId>pipeline-model-definition</artifactId>