oneDNN: fix build on darwin (#385054)

pkgs/development/libraries/oneDNN/default.nix

@@ -2,6 +2,7 @@
 , fetchFromGitHub
 , lib
 , stdenv
+, llvmPackages
 }:
 
 # This was originally called mkl-dnn, then it was renamed to dnnl, and it has
@@ -22,6 +23,8 @@ stdenv.mkDerivation (finalAttrs: {
 
   nativeBuildInputs = [ cmake ];
 
+  buildInputs = lib.optionals stdenv.hostPlatform.isDarwin [ llvmPackages.openmp ];
+
   # Tests fail on some Hydra builders, because they do not support SSE4.2.
   doCheck = false;