ulex: enable mac building

I've tested that ulex builds (and works) on OSX, and see no reason
it would not do so on other platforms, so I'm lifting the linux
restriction in platforms.

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

···
     homepage = http://www.cduce.org/download.html;
     description = "A lexer generator for Unicode and OCaml";
     license = stdenv.lib.licenses.mit;
-    platforms = stdenv.lib.platforms.linux;
+    platforms = ocaml.meta.platforms;
     maintainers = [ stdenv.lib.maintainers.roconnor ];
   };
 }