QCalendarBackend: disable copying

Silences clazy-copyable-polymorphic.

The Q_DISABLE_COPY_MOVE() disables the implicit default constructor,
so bring it back with =default.

Amends aa8393c94f.

Pick-to: 6.7 6.6 6.5
Change-Id: I44f0b19208d3865fb84e277bcaf65d7f4541d368
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/time/qcalendarbackend_p.h

@@ -59,8 +59,10 @@ class Q_CORE_EXPORT QCalendarBackend
 {
     friend class QCalendar;
     friend class QtPrivate::QCalendarRegistry;
+    Q_DISABLE_COPY_MOVE(QCalendarBackend)
 
 public:
+    QCalendarBackend() = default;
     virtual ~QCalendarBackend();
     virtual QString name() const = 0;