Add the "We mean it" statement to private header

There are more private headers with the statement but they are auto
generated using qdbusxml2cpp and should match whatever was generated.

Change-Id: I863f3703fc01fdd827dd9ccc480179880ae0d193
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>

src/bluetooth/qleadvertiser_p.h

@@ -34,6 +34,17 @@
 #ifndef QLEADVERTISER_P_H
 #define QLEADVERTISER_P_H
 
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
 #include "qlowenergyadvertisingdata.h"
 #include "qlowenergyadvertisingparameters.h"