iOS: Mark Scheduler class as final

Summary:
This fixes some build flavor:

```
error: destructor called on non-final 'facebook::react::Scheduler' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-virtual-dtor]
```

Reviewed By: shergin

Differential Revision: D7958022

fbshipit-source-id: 6ff64bdaa221b5c6430a98244d40d6d3789ba937

Author: fkgozali

ReactCommon/fabric/uimanager/Scheduler.h

@@ -21,7 +21,7 @@ class FabricUIManager;
 /*
  * Scheduler coordinates Shadow Tree updates and event flows.
  */
-class Scheduler:
+class Scheduler final:
   public UIManagerDelegate,
   public ShadowTreeDelegate {