Fix improper ELF versions of certain symbols

They got marked as Qt6_PRIVATE_API because of these private header
defining classes that don't follow Qt's naming convention ("mutex",
"condition_variable", "ControlElement"). We have an exclusion for
classes whose name start with Q.

Introduced by 5d903a64aca37ee7c2836e479e175336e9b7ca87 in the old Perl
syncqt and ported over.

Fixes: QTBUG-109605
Fixes: QTBUG-109604
Change-Id: I69ecc04064514f939896fffd173369623c960bcc
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
(cherry picked from commit 41b38c802b2b6251bdd65f8bf0d2031c304d70f6)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/thread/qwaitcondition_p.h

@@ -19,6 +19,11 @@
 #include <QtCore/QDeadlineTimer>
 #include <QtCore/private/qglobal_p.h>
 
+// This header always defines a class called "mutex" and one called
+// "condition_variable", so those mustn't be used to mark ELF symbol
+// visibility. Don't add more classes to this header!
+// ELFVERSION:stop
+
 #include <condition_variable>
 #include <mutex>
 

src/widgets/widgets/qmdisubwindow_p.h

@@ -39,7 +39,7 @@ class QMouseEvent;
 
 namespace QMdi {
 template<typename T>
-class ControlElement : public T
+class ControlElement : public T             // ELFVERSION:ignore
 {
 public:
     ControlElement(QMdiSubWindow *child) : T(child, nullptr)
@@ -58,7 +58,7 @@ public:
     QPointer<QMdiSubWindow> mdiChild;
 };
 
-class ControlContainer : public QObject
+class ControlContainer : public QObject     // ELFVERSION:ignore
 {
 public:
     ControlContainer(QMdiSubWindow *mdiChild);