kconfig: gconf: use gtk_dialog_get_content_area() accessor

GTK 3 removes many implementation details and struct members from its
public headers.

Use the gtk_check_menu_item_get_active() accessor.

[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/2.24.33/docs/reference/gtk/compiling.sgml#L85



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>