fix warning in libsoup
https://gitlab.gnome.org/GNOME/vala/blob/master/vapi/libsoup-2.4.vapi#L149
warning: Creation method of abstract class cannot be public.
public Auth (GLib.Type type, Soup.Message msg, string auth_header);
https://gitlab.gnome.org/GNOME/vala/blob/master/vapi/libsoup-2.4.vapi#L149
warning: Creation method of abstract class cannot be public.
public Auth (GLib.Type type, Soup.Message msg, string auth_header);