Fix -Wunused-but-set-variable in QTcpSocket documentation snippet

Pick-to: 6.2 6.3 5.15
Task-number: QTBUG-101883
Change-Id: I54dc7229565cef199f306a0ac837a932aeb00ec6
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/network/doc/snippets/network/tcpwait.cpp

@@ -50,7 +50,7 @@
 
 #include <QTcpSocket>
 
-void test_tcpwait()
+int test_tcpwait()
 {
     QTcpSocket socket;
     socket.connectToHost("localhost", 1025);
@@ -69,4 +69,5 @@ void test_tcpwait()
             break;
     }
 //! [0]
+    return numReadTotal;
 }