Specify correct type of joinClasses

Summary:
The type of `joinClasses` in fbjs 1.0 which draft-js open-source uses is incorrect (it is correct in www), specify the correct type when importing.

I would have thought that I would have to cast the import to `any` before casting it to a different type, but apprently this works (tested in cloned github repo).

Reviewed By: claudiopro

Differential Revision: D14318804

fbshipit-source-id: e4fd34ca5efda6fd9be681a49905d9e9fdb9e3ae

Author: gkz

src/component/contents/DraftEditorContents-core.react.js

@@ -22,7 +22,10 @@ const DraftOffsetKey = require('DraftOffsetKey');
 const React = require('React');
 
 const cx = require('cx');
-const joinClasses = require('joinClasses');
+const joinClasses: (
+  className?: ?string,
+  ...classes: Array<?string>
+) => string = require('joinClasses');
 const nullthrows = require('nullthrows');
 
 type Props = {