Exclude helm files from Apache Rat Check.

*Motivation*

Some of the helm files (e.g. helm ignore file and template file) doesn't have ASF license header.
It is tricky to add license headers for them. so exclude them from rat check.

*Changes*

Exclude two files from Apache Rat Check
diff --git a/pom.xml b/pom.xml
index 40c9a10..0a2fae5 100644
--- a/pom.xml
+++ b/pom.xml
@@ -1113,6 +1113,10 @@
 
             <!-- Configuration Templates -->
             <exclude>conf/schema_example.conf</exclude>
+
+            <!-- helm files -->
+            <exclude>**/.helmignore</exclude>
+            <exclude>**/_helpers.tpl</exclude>
           </excludes>
         </configuration>
       </plugin>