Fix use of QtWebKit in QtCreator's Help plugin

The help plugin uses the widgets API of QtWebKit and therefore needs
QtWebKitWidgets

Change-Id: I49b8530e3f44b42c48979c33d0b6b1fdb20c374b
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/help/help.pro

@@ -4,7 +4,7 @@ TARGET = Help
 QT += network
 greaterThan(QT_MAJOR_VERSION, 4) {
     QT += printsupport help
-    !isEmpty(QT.webkit.name): QT += webkit
+    !isEmpty(QT.webkitwidgets.name): QT += webkitwidgets
     else: DEFINES += QT_NO_WEBKIT
 } else {
     CONFIG += help