Fixed a pathname separator issue on Mac OS X.
Class FileSystemFilter was sometimes using a backslash as pathname separator even on Mac OS X, where that character is not allowed in pathnames.
git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@630 67173fc5-114c-0410-ac8e-9d2fd5bffc1f