Fix compiling with _IRR_WCHAR_FILESYSTEM on MinGW. Thx @  alexzk for reporting. Note that I tested this with gcc 4.7.0 and only compiling is tested.
Also added some documentation that UNICODE must be set as well for compiling with _IRR_WCHAR_FILESYSTEM ('cause I always forget to set that at first otherwise...).

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4322 dfc29bdd-3216-0410-991c-e03cc46cb475