Update webpack sample: Fix prop-types usage

This got missed in the initial update to React 16. It would be nice to have an automated regression test for the samples to catch oversights like this in the future.

Author: dustinsoftware

src/React.Sample.Webpack/Content/components/Avatar.jsx

@@ -8,6 +8,7 @@
  */
 
 var React = require('react');
+var PropTypes = require('prop-types');
 
 class Avatar extends React.Component {
 	static propTypes = {

src/React.Sample.Webpack/Content/components/Comment.jsx

@@ -9,6 +9,7 @@
 
 var Avatar = require('./Avatar');
 var React = require('react');
+var PropTypes = require('prop-types');
 
 class Comment extends React.Component {
 	static propTypes = {

src/React.Sample.Webpack/Content/components/CommentsBox.jsx

@@ -9,6 +9,7 @@
 
 var Comment = require('./Comment');
 var React = require('react');
+var PropTypes = require('prop-types');
 
 class CommentsBox extends React.Component {
 	static propTypes = {