ocaml-modules: replace buildInputs with nativeBuildInputs where appropriate

The default has been to use buildInputs for build dependencies.
This doesn't work when cross-compiling.

pkgs/development/ocaml-modules/ounit/default.nix

@@ -14,7 +14,9 @@ stdenv.mkDerivation {
     sha256 = "0hbd2sqdz75lv5ax82yhsfdk1dlcvq12xpys6n85ysmrl0c3d3lk";
   });
 
-  buildInputs = [ ocaml findlib ocamlbuild ];
+  nativeBuildInputs = [ ocaml findlib ocamlbuild ];
+  buildInputs = [ findlib ];
+  configurePlatforms = [];
 
   dontAddPrefix = true;