YARN-11435. Fix CheckStyle.

Author: slfan1989

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-common/src/main/java/org/apache/hadoop/yarn/server/federation/utils/FederationStateStoreFacade.java

@@ -105,7 +105,7 @@ public final class FederationStateStoreFacade {
   private static final Logger LOG =
       LoggerFactory.getLogger(FederationStateStoreFacade.class);
 
-  private static FederationStateStoreFacade facade;
+  private static volatile FederationStateStoreFacade facade;
 
   private static Random rand = new Random(System.currentTimeMillis());