launch-new-instance: Fix warning

Commit 2667b9f3e5 updated the extension for a shell change that
renamed _onActivate() to _activate, but forgot to also rename the
declaration of the variable that stores the original function.
Fix this to stop a warning about assignment to an undefined variable.

https://bugzilla.gnome.org/show_bug.cgi?id=745470

extensions/launch-new-instance/extension.js

@@ -1,6 +1,6 @@
 const AppDisplay = imports.ui.appDisplay;
 
-var _onActivateOriginal = null;
+let _activateOriginal = null;
 
 function init() {
 }