gconstructor.h: Visual Studio: Only include gslist.h if needed

We only want to include gslist.h here if it was not already included via
including glib.h, as:

* gslist.h should normally be included via glib.h if used outside of
  GLib itself.
* This broke Visual Studio builds that use GResources (via
  glib-compile-resources.exe) as that would cause the generated code to
  include gslist.h directly, which is therefore disallowed.