add a cvsignore file that ignores the file custom_config_inc.php

tired of seeing the question mark every time I update :)

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1299 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9