SDL2_ttf: fix cross

pkgs/development/libraries/SDL2_ttf/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, darwin, fetchurl, SDL2, freetype, libGL }:
+{ lib, stdenv, pkg-config, darwin, fetchurl, SDL2, freetype, libGL }:
 
 stdenv.mkDerivation rec {
   pname = "SDL2_ttf";
@@ -11,6 +11,8 @@ stdenv.mkDerivation rec {
 
   configureFlags = lib.optional stdenv.isDarwin "--disable-sdltest";
 
+  nativeBuildInputs = [ pkg-config ];
+
   buildInputs = [ SDL2 freetype libGL ]
     ++ lib.optional stdenv.isDarwin darwin.libobjc;