Add definition for React DOM SyntheticPointerEvent

This is based on facebook/react#12507 which is
seemingly missing a couple of fields from the official spec.
Consequently this will need an update once React supports all fields.

lib/react-dom.js

@@ -470,7 +470,7 @@ type ReactDOM$SVGElementProps = ReactDOM$ElementProps & {
   xmlBase?: string,
   xmlLang?: string,
   xmlSpace?: string
-  
+
   // TODO: SVG presentation props
   // TODO: SVG filters props
   // TODO: SVG animation props