Fix warning about not calling the base class copy constructor

error: base class ‘class QBasicAtomicPointer<void>’ should be explicitly initialized in the copy constructor [-Werror=extra]

Change-Id: I2bc52f3c7a574209b213fffd149b4b71f3006be5
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/thread/qatomic.h

@@ -176,6 +176,9 @@ public:
     }
 #endif
     inline QAtomicPointer(const QAtomicPointer<T> &other) Q_DECL_NOTHROW
+#ifdef QT_BASIC_ATOMIC_HAS_CONSTRUCTORS
+        : QBasicAtomicPointer<T>()
+#endif
     {
         this->storeRelease(other.loadAcquire());
     }