Add "We mean it" header

Also use canonical Qt header include.

Change-Id: I5c1133fc088ef3e29d3336591bc1f9a98868db8c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/testlib/qtestregistry_p.h

@@ -4,7 +4,18 @@
 #ifndef QTESTREGISTRY_P_H
 #define QTESTREGISTRY_P_H
 
-#include <QString>
+//
+//  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 <QtCore/qstring.h>
 #include <QtCore/qhash.h>
 #include <QtTest/qttestglobal.h>