libgimp: fix a type cast warning (win32).

I had this warning when cross-building for Windows 64-bit:

> libgimp/gimpui.c:187:52: warning: passing argument 2 of
> 'gdk_win32_window_foreign_new_for_display' makes pointer from integer
> without a cast [-Wint-conversion]

> note: expected 'HWND' {aka 'struct HWND__ *'} but argument is of type 'guint32' {aka 'unsigned int'}

libgimp/gimpui.c

@@ -184,7 +184,7 @@ gimp_ui_get_foreign_window (guint32 window)
 
 #ifdef GDK_WINDOWING_WIN32
   return gdk_win32_window_foreign_new_for_display (gdk_display_get_default (),
-                                                   window);
+                                                   (HWND) (uintptr_t) window);
 #endif
 
   return NULL;