kconfig: gconf: always destroy dialog in on_window1_delete_event()

kconfig: gconf: always destroy dialog in on_window1_delete_event()

When gtk_dialog_run() returns GTK_RESPONSE_YES or GTK_RESPONSE_NO,
gtk_widget_destroy() is not called, resulting in a memory leak.

It is better to always destroy the dialog, even if the application
is about to exit.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>