fix DefinePlugin stringifyObj bug

Author: kagawagao

lib/DefinePlugin.js

@@ -35,10 +35,10 @@ class DefinePlugin {
 				}(definitions, ""));
 
 				function stringifyObj(obj) {
-					return "{" + Object.keys(obj).map((key) => {
+					return "({" + Object.keys(obj).map((key) => {
 						let code = obj[key];
 						return JSON.stringify(key) + ":" + toCode(code);
-					}).join(",") + "}";
+					}).join(",") + "})";
 				}
 
 				function toCode(code) {