Fix builds using QQuickWebEngineView

We need to include the definition for Q_WEBVIEW_EXPORT for
the QQuickWebEngineView implementation.

Change-Id: Ia5b74fc757aa6e1e584e45063a038ffe77f971c0
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>