Add dep to get the root global variable

Author: kgryte

package.json

@@ -22,7 +22,8 @@
     "url": "https://github.com/stdlib-js/stdlib/issues"
   },
   "dependencies": {
-    "object-keys": "^1.0.9"
+    "object-keys": "^1.0.9",
+    "system.global": "^1.0.0"
   },
   "devDependencies": {
     "browserify": "13.x.x",