gnubg: fix build

Presumably, readline used to be propagated from one of the other inputs
but is no longer (the build succeeds on 16.09 but is unable to locate
readline.h on master).

https://hydra.nixos.org/build/42761215

pkgs/games/gnubg/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pkgconfig, glib, python, gtk2 }:
+{ stdenv, fetchurl, pkgconfig, glib, python, gtk2, readline }:
 
 let version = "1.04.000"; in
 stdenv.mkDerivation {
@@ -9,7 +9,7 @@ stdenv.mkDerivation {
     sha256 = "0gsfl6qbj529d1jg3bkyj9m7bvb566wd7pq5fslgg5yn6c6rbjk6";
   };
 
-  buildInputs = [ pkgconfig python glib gtk2 ];
+  buildInputs = [ pkgconfig python glib gtk2 readline ];
 
   configureFlags = [ "--with-gtk" "--with--board3d" ];