HADOOP-15287. JDK9 JavaDoc build fails due to one-character underscore identifiers in hadoop-yarn-common

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/pom.xml

@@ -371,6 +371,12 @@
               </excludes>
             </configuration>
           </plugin>
+          <plugin>
+            <artifactId>maven-javadoc-plugin</artifactId>
+            <configuration>
+              <excludePackageNames>org.apache.hadoop.yarn.webapp.hamlet</excludePackageNames>
+            </configuration>
+          </plugin>
         </plugins>
       </build>
     </profile>