Fix warnings about unused private fields we can't remove

These are in public classes, so we can't remove them until we get to
Qt 6.

qtimer.h:231:20: warning: private field 'del' is not used [-Wunused-private-field]

Change-Id: I84e363d735b443cb9beefffd14b8b5fb720c9e41
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/kernel/qtimer.cpp

@@ -148,6 +148,7 @@ static const int INV_TIMER = -1;                // invalid timer id
 QTimer::QTimer(QObject *parent)
     : QObject(parent), id(INV_TIMER), inter(0), del(0), single(0), nulltimer(0), type(Qt::CoarseTimer)
 {
+    Q_UNUSED(del);  // ### Qt 6: remove field
 }