Fix build of html2pdf example

Add the missing QTextStream include.

Change-Id: I892a90deaccde03df3d974c35b6d41dcc013ef65
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>

examples/webenginewidgets/html2pdf/html2pdf.cpp

@@ -41,6 +41,7 @@
 #include <QApplication>
 #include <QCommandLineParser>
 #include <QFile>
+#include <QTextStream>
 #include <QWebEnginePage>
 
 #include <functional>