kconfig: gconf: replace GdkColor with GdkRGBA

kconfig: gconf: replace GdkColor with GdkRGBA

GdkColor is deprecated with GTK 3.14. [1]

Use GdkRGBA instead.

This fixes warnings such as:

scripts/kconfig/gconf.c: In function ‘set_node’:
scripts/kconfig/gconf.c:138:9: warning: ‘gdk_color_parse’ is deprecated: Use 'gdk_rgba_parse' instead [-Wdeprecated-declarations]
  138 |         gdk_color_parse(menu_is_visible(menu) ? "Black" : "DarkGray", &color);
      |         ^~~~~~~~~~~~~~~

[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/3.14.0/gdk/deprecated/gdkcolor.h#L52

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