Add "We mean it" warning to new private header

Amends 5e48a51608, fixing warning
from syncqt.

Change-Id: I15d22a1bba865777c17180438b6e89ee9e1ae78b
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/tools/qatomicscopedvaluerollback_p.h

@@ -4,6 +4,17 @@
 #ifndef QATOMICSCOPEDVALUEROLLBACK_P_H
 #define QATOMICSCOPEDVALUEROLLBACK_P_H
 
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists for the convenience
+// of qapplication_*.cpp, qwidget*.cpp and qfiledialog.cpp.  This header
+// file may change from version to version without notice, or even be removed.
+//
+// We mean it.
+//
+
 #include <QtCore/qglobal.h>
 #include <QtCore/qatomic.h>