[MCHECKSTYLE-184] Allow src/main/resources to be searched by Checkstyle

Now that we let Checkstyle look at properties files, we need to make sure that the RegexpHeader check only looks at java files.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1441961 13f79535-47bb-0310-9956-ffa450edef68