nixpkgs: add pkgconfig to icestorm nativeBuildInputs

This helps iceprog find libftdi appropriately on its own, without a
bunch of gross errors in the build process (which should not be a
problem, but may *look* like one!)

Signed-off-by: Austin Seipp <[e-mail]>

pkgs/development/tools/icestorm/default.nix

···
-{ stdenv, fetchFromGitHub, python3, libftdi }:
+{ stdenv, fetchFromGitHub, python3, libftdi, pkgconfig }:
 
 stdenv.mkDerivation rec {
   name = "icestorm-${version}";
···
     sha256 = "18qy7gylnydgzmqry1b4r0ilm6lkjdcyn0wj03syxdig9dbjiacm";
   };
 
+  nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ python3 libftdi ];
   preBuild = ''
     makeFlags="PREFIX=$out $makeFlags"