msgpack-tools: include darwin

pkgs/development/tools/msgpack-tools/default.nix

···
     description = "Command-line tools for converting between MessagePack and JSON";
     homepage = "https://github.com/ludocode/msgpack-tools";
     license = licenses.mit;
-    platforms = platforms.linux;
+    platforms = platforms.linux ++ platforms.darwin;
     maintainers = with maintainers; [ alibabzo ];
   };
 }