Add warning about native interface APIs to platform headers

Those platform specific classes are not covered by our
binary/source compatibility guarantee between minor releases.

Pick-to: 6.3
Change-Id: I2a69c7ea07feea62fdf9899839e47fc540f89c12
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qcoreapplication_platform.h

@@ -40,6 +40,15 @@
 #ifndef QCOREAPPLICATION_PLATFORM_H
 #define QCOREAPPLICATION_PLATFORM_H
 
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is part of the native interface APIs. Usage of
+// this API may make your code source and binary incompatible
+// with future versions of Qt.
+//
+
 #include <QtCore/qglobal.h>
 #include <QtCore/qnativeinterface.h>
 #include <QtCore/qcoreapplication.h>

src/gui/kernel/qguiapplication_platform.h

@@ -40,6 +40,15 @@
 #ifndef QGUIAPPLICATION_PLATFORM_H
 #define QGUIAPPLICATION_PLATFORM_H
 
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is part of the native interface APIs. Usage of
+// this API may make your code source and binary incompatible
+// with future versions of Qt.
+//
+
 #include <QtGui/qtguiglobal.h>
 
 #include <QtCore/qnativeinterface.h>

src/gui/kernel/qoffscreensurface_platform.h

@@ -40,6 +40,15 @@
 #ifndef QOFFSCREENSURFACE_PLATFORM_H
 #define QOFFSCREENSURFACE_PLATFORM_H
 
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is part of the native interface APIs. Usage of
+// this API may make your code source and binary incompatible
+// with future versions of Qt.
+//
+
 #include <QtGui/qtguiglobal.h>
 #include <QtGui/qoffscreensurface.h>
 #include <QtCore/qnativeinterface.h>

src/gui/kernel/qopenglcontext_platform.h

@@ -40,6 +40,15 @@
 #ifndef QOPENGLCONTEXT_PLATFORM_H
 #define QOPENGLCONTEXT_PLATFORM_H
 
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is part of the native interface APIs. Usage of
+// this API may make your code source and binary incompatible
+// with future versions of Qt.
+//
+
 #ifndef QT_NO_OPENGL
 
 #include <QtGui/qtguiglobal.h>