libunistring: fix sdk version

As per the discussion at:
https://github.com/NixOS/nixpkgs/pull/291282#discussion_r1668442573
mixing SDK versions breaks building on x86_64-darwin

Co-authored-by: reckenrode <randy@largeandhighquality.com>

pkgs/development/libraries/libunistring/default.nix

@@ -31,7 +31,7 @@ stdenv.mkDerivation (finalAttrs: {
   strictDeps = true;
   propagatedBuildInputs = lib.optional (!stdenv.isLinux) libiconv;
   buildInputs = lib.optionals stdenv.isDarwin [
-    darwin.apple_sdk_11_0.frameworks.CoreServices
+    darwin.apple_sdk.frameworks.CoreServices
   ];
   nativeBuildInputs = [ updateAutotoolsGnuConfigScriptsHook ];