Do not include windows_cconv.h on non mingw32 systems

Do not include windows_cconv.h on non windows systems, or else build
fails with `#error Unknown mingw32 arch` on architectures other than
i386 or x86_64.

Author: iliastsi

GUI/GtkExtras.hs

@@ -15,13 +15,13 @@ import Foreign
 import Foreign.C
 import Control.Concurrent.MVar
 
-#if !(mingw32_HOST_OS || mingw32_TARGET_OS)
+#if mingw32_HOST_OS || mingw32_TARGET_OS
+#include "windows_cconv.h"
+#else
 import System.Glib.GError
 import Control.Monad
 #endif
 
-#include "windows_cconv.h"
-
 waitGUI :: IO ()
 waitGUI = do
   resultVar <- newEmptyMVar