YARN-6200. addendum to fix compilation error caused by reverting YARN-5068.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/amlauncher/AMLauncher.java

@@ -51,6 +51,7 @@ import org.apache.hadoop.yarn.api.records.ContainerId;
 import org.apache.hadoop.yarn.api.records.ContainerLaunchContext;
 import org.apache.hadoop.yarn.api.records.NodeId;
 import org.apache.hadoop.yarn.client.NMProxy;
+import org.apache.hadoop.yarn.conf.YarnConfiguration;
 import org.apache.hadoop.yarn.event.EventHandler;
 import org.apache.hadoop.yarn.exceptions.YarnException;
 import org.apache.hadoop.yarn.ipc.YarnRPC;