Add missing license headers found with RAT

yarn/pom.xml

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- ~ Licensed to the Apache Software Foundation (ASF) under one or more
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one or more
   ~ contributor license agreements.  See the NOTICE file distributed with
   ~ this work for additional information regarding copyright ownership.
   ~ The ASF licenses this file to You under the Apache License, Version 2.0
@@ -14,7 +15,6 @@
   ~ See the License for the specific language governing permissions and
   ~ limitations under the License.
   -->
-
 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <parent>