Put qxmlstreamreader code behind its feature-flag

Missed feature guards from:
- QXmlStreamReaderPrivate class declaration
- QXmlStreamWriter::writeCurrentToken definition [*]

[*] Function takes in a qxmlstreamreader. The declaration
of the function already had the necessary feature guard.

Pick-to: 6.8
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>

src/corelib/serialization/qxmlstream.cpp

@@ -3880,6 +3880,7 @@ void QXmlStreamWriterPrivate::writeStartElement(QAnyStringView namespaceUri, QAn
     didWriteAnyToken = true;
 }
 
+#if QT_CONFIG(xmlstreamreader)
 /*!  Writes the current state of the \a reader. All possible valid
   states are supported.
 
@@ -3943,8 +3944,9 @@ void QXmlStreamWriter::writeCurrentToken(const QXmlStreamReader &reader)
         break;
     }
 }
-
+#endif // feature xmlstreamreader
 #endif // feature xmlstreamwriter
+
 #if QT_CONFIG(xmlstreamreader)
 static constexpr bool isTokenAllowedInContext(QXmlStreamReader::TokenType type,
                                                QXmlStreamReaderPrivate::XmlContext ctxt)

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