It seems that since GjsPrivate-1.0.[gir|typelib] refers to APIs from
there, we ought to export those as well for the Visual Studio builds via
GJS_EXPORT, so that they can be loaded by the girepository DLL.
It turns out that this portion of the code used to be a DLL/.so of its own,
so it also makes sense to export these so that they are available from
gjs.lib as well.