mixin definition constructor init

lib/less/node/mixin.js

@@ -20,3 +20,8 @@ tree.mixin.Call.prototype = {
         throw new(Error)("mixin " + this.selector.toCSS() + " is undefined");
     }
 };
+
+tree.mixin.Definition = function MixinDefinition(name, params) {
+    this.name = name;
+    this.params = params;
+};