gv: add explicit libXext build depend

A preparation to remove xlibsWrapper from Xaw3d's propagatedbuildInputs.

pkgs/applications/misc/gv/default.nix

···
-{ lib, stdenv, fetchurl, Xaw3d, ghostscriptX, perl, pkg-config, libiconv }:
+{ lib, stdenv, fetchurl, libXext, Xaw3d, ghostscriptX, perl, pkg-config, libiconv }:
 
 stdenv.mkDerivation rec {
   pname = "gv";
···
 
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [
+    libXext
     Xaw3d
     ghostscriptX
     perl