[introspection] Use json-gobject.h
[introspection] Use json-gobject.h

The json-gobject.h header file includes json-glib.h and all the
GObject-related serialization and deserialization API; we assume
that if you want introspection you should be using this header
and not the plain json-glib.h.

robtaylor/json-glib-gvariant,GNOME/json-glib,oerdnj/json-glib,Distrotech/json-glib,ebassi/json-glib,oerdnj/json-glib,Distrotech/json-glib,brauliobo/json-glib,frida/json-glib,ebassi/json-glib,brauliobo/json-glib,robtaylor/json-glib-gvariant,frida/json-glib,brauliobo/json-glib,GNOME/json-glib,ebassi/json-glib,Distrotech/json-glib,oerdnj/json-glib,oerdnj/json-glib