Export QFutureCallOutInterface to allow external use

2dea20e4b0 de-inlined the dtor to avoid
duplicating its vtable in multiple translation units, but the class is
used externally, so we need to export it now.

Pick-to: 6.3
Fixes: QTBUG-100569
Change-Id: I7cf2abdfdeb59bced0631838fe329ba94ab8c73a
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/thread/qfutureinterface_p.h

@@ -122,7 +122,7 @@ private:
     { }
 };
 
-class QFutureCallOutInterface
+class Q_CORE_EXPORT QFutureCallOutInterface
 {
 public:
     virtual ~QFutureCallOutInterface();