Put qxmlstreamreader code behind its feature-flag

Missed feature guards from:
- QXmlStreamReaderPrivate class declaration

(Cherry-pick edit for 6.8: writeCurrentToken guards
already existed)

Fixes: QTBUG-135230
Change-Id: I78741bc75a6baa8bc86ec5ce7d34a4333d8bdad4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 90c90d50715b7e7501b0a2153122d576656b9e4d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 3e67c1241a1915effd96d9c5b2d797f7a5d7df2a)
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/serialization/qxmlstream_p.h

@@ -225,7 +225,7 @@ public:
     }
 };
 
-
+#if QT_CONFIG(xmlstreamreader)
 class QXmlStreamEntityResolver;
 class QXmlStreamReaderPrivate : public QXmlStreamGrammar, public QXmlStreamPrivateTagStack
 {
@@ -535,6 +535,7 @@ private:
             type = t;
     }
 };
+#endif // feature xmlstreamreader
 
 QT_END_NAMESPACE