Add flow pragma to 'Draft.js' (#1262)

**what is the change?:**
Adds missing pragma.

**why make this change?:**
This was causing a problem for users in some build set-ups.
Fixes #982
Thanks @yuku-t for this solution. :)

**test plan:**
Ran `yarn build` and inspected `lib/Draft.js.flow` before and after this
fix. It has the `@flow` pragma after, and did not before.

**issue:**
#982

src/Draft.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule Draft
+ * @flow
  */
 
 'use strict';