kconfig: gconf: replace GDK_space with GDK_KEY_space

In GTK3, keysyms changed to have a KEY_ prefix.

[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/2.24.33/gdk/gdkkeysyms-compat.h#L24



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