SDL2_net: fix cross

pkgs/development/libraries/SDL2_net/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, darwin, fetchurl, SDL2 }:
+{ lib, stdenv, pkg-config, darwin, fetchurl, SDL2 }:
 
 stdenv.mkDerivation rec {
   pname = "SDL2_net";
@@ -9,6 +9,8 @@ stdenv.mkDerivation rec {
     sha256 = "08cxc1bicmyk89kiks7izw1rlx5ng5n6xpy8fy0zxni3b9z8mkhm";
   };
 
+  nativeBuildInputs = [ pkg-config ];
+
   buildInputs = lib.optional stdenv.isDarwin darwin.libobjc;
 
   configureFlags = lib.optional stdenv.isDarwin "--disable-sdltest";