kconfig: gconf: replace GTK_STOCK_{OK,NO,CANCEL}

These are deprecated with GTK 3.10. [1]

Use "_OK", "_no", "_Cancel".

[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/3.10.0/gtk/deprecated/gtkstock.h#L827



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