qrestreply_p.h: include what you need

QStringDecoder within std::optional needs QStringDecoder to be fully
defined ("in-size" (Lakos)).

QByteArray, used as a function parameter, can be forward-declard,
though ("in-name-only" (Lakos)).

This also makes the header comply with Qt's private symbol versioning,
which requires every private header to include some other private
header, or, eventually, qglobal_p.h.

Amends 4da14a67a6.

Pick-to: 6.7
Change-Id: I35fed05955f1194299162b576834c1045672bcac
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/network/access/qrestreply_p.h

@@ -15,12 +15,14 @@
 // We mean it.
 //
 
+#include <QtCore/private/qstringconverter_p.h>
+
 #include <optional>
 
 QT_BEGIN_NAMESPACE
 
+class QByteArray;
 class QNetworkReply;
-class QStringDecoder;
 
 class QRestReplyPrivate
 {