Include gdkwin32.h (which includes gdkprivate-win32.h and gdkwin32cursor.h
during the build of GDK-Win32) so that
gdk_win32_icon_to_pixbuf_libgtk_only() and
gdk_win32_pixbuf_to_hicon_libgtk_only() get exported, so that the GTK
DLL can link correctly.