[WIN32] replace old dirent emulation with a more complete one.

I can't find a reference to dirent anymore but I feel better if we have a working version included. Just as a pr as I dunno if I can use dirent.h without restrictions in our project.
https://github.com/wsoltys/xbmc/blob/dirent/xbmc/win32/dirent.h

Thanks fritsch for pointing that out :)