GdkSurface - Don't call unimplemented NULL set_startup_id() vfunc on Win32

GdkSurface - Don't call unimplemented NULL set_startup_id() vfunc on Win32

GdkSurface::set_startup_id() is NULL on Win32 and would cause a segfault
if called.

While the documentation of the main caller of set_startup_id(),
gtk_window_set_startup_id(), mentions that it's not implemented on
Windows it can still be automatically called via Glade and simply doing
nothing on Win32 is going to be less disruptive than a segfault.