tst_QThread: Unbreak UBSan build

tst_QThread peeks into QThreadPrivate, which means that a UBSan build
needs access to QThreadPrivate's type_info, for which we need to
export the class.

Amends 268ff00ef5.

Pick-to: 6.6 6.5
Change-Id: Ic26df3d323d50b51d369d5f2bd78db7e047b5341
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthread_p.h

@@ -170,7 +170,7 @@ public:
     ~QDaemonThread();
 };
 
-class QThreadPrivate : public QObjectPrivate
+class Q_AUTOTEST_EXPORT QThreadPrivate : public QObjectPrivate
 {
     Q_DECLARE_PUBLIC(QThread)