smlpkg: add platforms

pkgs/tools/package-management/smlpkg/default.nix

···
     description = "Generic package manager for Standard ML libraries and programs";
     homepage = "https://github.com/diku-dk/smlpkg";
     license = licenses.mit;
+    platforms = mlton.meta.platforms;
     maintainers = with maintainers; [ athas ];
   };
 }