codegen: Don't emit g_type_add_instance_private() in *_register_type()
codegen: Don't emit g_type_add_instance_private() in *_register_type()

This fixes warnings like "sys:1: Warning: cannot add private field to
invalid (non-instantiatable) type ..."

Regression of df56f916e52e5efe3876afed59a6190f6cead1c8

GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,frida/vala