FindBugs fix
- fixed "Should be a static inner class"
- see http://findbugs.sourceforge.net/bugDescriptions.html#SIC_INNER_SHOULD_BE_STATIC


git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1568178 13f79535-47bb-0310-9956-ffa450edef68