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

Hi,

From the commit message:

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.

This will fix issues building items that depends on GResources that use glib-compile-resources.exe with Visual Studio, such as 1.

Fixes: libadwaita#418 (closed)

With blessings, thank you!