fix to undefined mixin error message

lib/less/tree/mixin.js

@@ -18,7 +18,7 @@ tree.mixin.Call.prototype = {
                 return rules;
             }
         }
-        throw new(Error)("mixin " + this.selector.toCSS() + " is undefined");
+        throw new(Error)(this.selector.toCSS().trim() + " is undefined");
     }
 };