HADOOP-19502. Add RegexpSingleline module to checkstyle.xml. (#7505) Contributed by hfutatzhanghb.

Signed-off-by: Shilun Fan <slfan1989@apache.org>

hadoop-build-tools/src/main/resources/checkstyle/checkstyle.xml

@@ -73,6 +73,11 @@
         <property name="max" value="100"/>
     </module>
 
+    <module name="RegexpSingleline">
+        <property name="format" value="\s+$"/>
+        <property name="severity" value="warn"/>
+    </module>
+
     <module name="TreeWalker">
 
         <module name="SuppressWarningsHolder"/>