This symbol needs to be exported for GDK (Win32) so that the
runtime checks for Win32 backend usage can be done on
MSVC-compiled versions of GTK+ too.
I did not add the corresponding symbols for the other backend
though-they are probably exported automatically by GCC AFAIK.
This is done to make commit
9db4accf9c30e29979e3e012f0a8b3ef624bb690
work on MSVC
gdk_win32_set_modal_dialog_libgtk_only
gdk_win32_window_is_win32
gdk_win32_window_get_handle
+gdk_win32_window_get_type
gdk_win32_display_get_type
gdk_win32_window_foreign_new_for_display
gdk_win32_window_lookup_for_display