Port file-flushing code to QFileDevice.

This fixes auto-flushing in the future QSaveFile class.

Change-Id: I6e84388070d5b9af9d326f5092ec9b55fd98cd05
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>

src/corelib/io/qtextstream.cpp

@@ -696,7 +696,7 @@ void QTextStreamPrivate::flushWriteBuffer()
 
     // flush the file
 #ifndef QT_NO_QOBJECT
-    QFile *file = qobject_cast<QFile *>(device);
+    QFileDevice *file = qobject_cast<QFileDevice *>(device);
     bool flushed = !file || file->flush();
 #else
     bool flushed = true;