Fix gtk on windows in ghci

As someone else pointed on gtk2hs mailing list (see http://permalink.gmane.org/gmane.comp.lang.haskell.gtk2hs/3171), ghci panics while loading gtk-0.12.5 on windows.

I'm not sure how safe is to remove manual _fmode setting on windows, but that was not present in previous version of gtk (0.12.4). It was added with one huge commit: https://github.com/gtk2hs/gtk2hs/commit/2ab8ab20102b73829d0b149ef16143916bab32da.