GUACAMOLE-136: Add Apache RAT plugin to build. Add missing license header to pom.xml.

extensions/guacamole-auth-duo/pom.xml

@@ -1,3 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+    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 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  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
@@ -134,6 +153,35 @@
                 </executions>
             </plugin>
 
+            <!-- Verify format using Apache RAT -->
+            <plugin>
+                <groupId>org.apache.rat</groupId>
+                <artifactId>apache-rat-plugin</artifactId>
+                <version>0.12</version>
+
+                <configuration>
+                    <excludes>
+                        <exclude>**/*.json</exclude>
+                        <exclude>src/licenses/**/*</exclude>
+                        <exclude>src/main/resources/templates/*.html</exclude>
+                        <exclude>src/main/resources/lib/DuoWeb/**/*</exclude>
+                        <exclude>src/main/java/com/duosecurity/duoweb/**/*</exclude>
+                    </excludes>
+                </configuration>
+
+                <!-- Bind RAT to validate phase -->
+                <executions>
+                    <execution>
+                        <id>validate</id>
+                        <phase>validate</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+
+            </plugin>
+
         </plugins>
     </build>