gram: fix darwin build

Hydra reports libclang to be missing, which is fixed by adding the
bindgen hook.

https://hydra.nixos.org/build/334182361

pkgs/by-name/gr/gram/package.nix

@@ -65,6 +65,7 @@ rustPlatform.buildRustPackage (finalAttrs: {
   ]
   ++ lib.optionals stdenv.hostPlatform.isDarwin [
     cargo-bundle
+    rustPlatform.bindgenHook
   ];
 
   dontUseCmakeConfigure = true;