QProcess: leave a warning about the signal with overload, for Qt 6

You can't write:
  connect(proc, &QProcess::finished, [](){});

because of the overload.

Change-Id: I651cc56ee15481392590dc44942d8e814fad75f6
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/corelib/io/qprocess.h

@@ -219,7 +219,7 @@ Q_SIGNALS:
         QPrivateSignal
 #endif
     );
-    void finished(int exitCode);
+    void finished(int exitCode); // ### Qt 6: merge the two signals with a default value
     void finished(int exitCode, QProcess::ExitStatus exitStatus);
     void error(QProcess::ProcessError error);
     void stateChanged(QProcess::ProcessState state