Ignore irrelevant header lines in checkstyle configuration

* Ignores the empty lines that may have trailing white-space in the
  license header. This allows formatting of the license header either
	with or without trailing white-space, which differs between different
	versions of Eclipse (as well as between IDEs).

Change-Id: If97d2cfe169accd6d05e44521c9ca5265d715772

checkstyle/vaadin-checkstyle.xml

@@ -90,6 +90,8 @@
     <module name="Header">
         <property name="headerFile" value="${checkstyle.header.file}" />
         <property name="fileExtensions" value="java" />
+        <!-- ignore empty lines that may or may not have trailing white-space -->
+        <property name="ignoreLines" value="3, 7, 9"/>
     </module>
 
     <module name="TreeWalker">