multistreamclient example: fix compilation

Says GCC:
  qtbase/examples/network/multistreamclient/timeconsumer.cpp:85:37: error: incomplete type ‘QIODevice’ used in nested name specifier
     85 |     QDataStream ds(&buf, QIODevice::WriteOnly);
        |                                     ^~~~~~~~~

Instead of including <QIODevice>, use the correct 'nested name
specifier', QIODeviceBase.

Pick-to: 6.2
Change-Id: Id8d87dbcb497e4feca1d5d3ce6b1bdb9da5c0dab
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

examples/network/multistreamclient/timeconsumer.cpp

@@ -82,7 +82,7 @@ void TimeConsumer::readDatagram(const QByteArray &ba)
 void TimeConsumer::timerTick()
 {
     QByteArray buf;
-    QDataStream ds(&buf, QIODevice::WriteOnly);
+    QDataStream ds(&buf, QIODeviceBase::WriteOnly);
 
     ds << lastTime;
     emit writeDatagram(buf);