Fix improper ELF versions of two more classes

They got marked as Qt6_PRIVATE_API because of these ARE private classes,
however their pointers are used in user code. For QMutexPrivate, because
we manipulate the pointer in inline code; for QPostEventList, because
it's used in a virtual member function's parameter list.

Pick-to: 6.5
Fixes: QTBUG-109605
Change-Id: I69ecc04064514f939896fffd1736d1fa8d3f1768
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/corelib/thread/qmutex_p.h

@@ -36,6 +36,8 @@ struct timespec;
 
 QT_BEGIN_NAMESPACE
 
+// We manipulate the pointer to this class in inline, atomic code,
+// so syncqt mustn't mark them as private, so ELFVERSION:ignore-next
 class QMutexPrivate
 {
 public:

src/corelib/thread/qthread_p.h

@@ -58,6 +58,7 @@ inline bool operator<(const QPostEvent &first, const QPostEvent &second)
 
 // This class holds the list of posted events.
 //  The list has to be kept sorted by priority
+// It's used in a virtual in QCoreApplication, so ELFVERSION:ignore-next
 class QPostEventList : public QList<QPostEvent>
 {
 public: