QFlags: add QT_NO_DATASTREAM checks to datastream operator declaration

Amends 729c0960b84183bcf9576f974cb5e3b3c3fdc6ef.

Found in Qt 6.9 API review.

Change-Id: Ieb81fe000e61fef54a003f52789fa1475aac5239
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 348b680ae56e92f061424a10c4baa726cf7f15d2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qflags.h

@@ -206,8 +206,10 @@ private:
         return (it == end ? Int(0) : (Int(*it) | initializer_list_helper(it + 1, end)));
     }
 
+#ifndef QT_NO_DATASTREAM
     template <typename E> friend QDataStream &operator<<(QDataStream &, QFlags<E>);
     template <typename E> friend QDataStream &operator>>(QDataStream &, QFlags<E> &);
+#endif
     using Base::i;
 };