Add missing QApplication include in streambookmarks example

Amends 3ccf2f8308.

Pick-to: 6.5 6.6
Change-Id: Ib51777ef9af44aff7fbb813b58eb0fbb74761373
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

examples/corelib/serialization/streambookmarks/mainwindow.cpp

@@ -17,7 +17,7 @@
 #  include <QClipboard>
 #endif
 #include <QDesktopServices>
-#include <QGuiApplication>
+#include <QApplication>
 #include <QScreen>
 
 using namespace Qt::StringLiterals;