iOS: Mark Scheduler class as final
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

exponent/react-native,exponentjs/react-native,arthuralee/react-native,hammerandchisel/react-native,facebook/react-native,pandiaraj44/react-native,janicduplessis/react-native,hoangpham95/react-native,arthuralee/react-native,exponent/react-native,pandiaraj44/react-native,myntra/react-native,janicduplessis/react-native,exponentjs/react-native,exponentjs/react-native,myntra/react-native,javache/react-native,exponent/react-native,hoangpham95/react-native,pandiaraj44/react-native,javache/react-native,exponentjs/react-native,hammerandchisel/react-native,exponent/react-native,hammerandchisel/react-native,pandiaraj44/react-native,hoangpham95/react-native,hoangpham95/react-native,janicduplessis/react-native,myntra/react-native,facebook/react-native,myntra/react-native,javache/react-native,javache/react-native,myntra/react-native,hoangpham95/react-native,pandiaraj44/react-native,janicduplessis/react-native,exponentjs/react-native,myntra/react-native,javache/react-native,javache/react-native,pandiaraj44/react-native,exponent/react-native,exponentjs/react-native,exponent/react-native,janicduplessis/react-native,hammerandchisel/react-native,facebook/react-native,exponentjs/react-native,pandiaraj44/react-native,arthuralee/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,exponent/react-native,facebook/react-native,janicduplessis/react-native,myntra/react-native,facebook/react-native,facebook/react-native,facebook/react-native,exponentjs/react-native,arthuralee/react-native,myntra/react-native,hammerandchisel/react-native,hoangpham95/react-native,arthuralee/react-native,hammerandchisel/react-native,javache/react-native,janicduplessis/react-native,hammerandchisel/react-native,myntra/react-native,exponent/react-native,pandiaraj44/react-native,hoangpham95/react-native,javache/react-native,hoangpham95/react-native,hammerandchisel/react-native,facebook/react-native