QMessageAuthenticationCode: make Private::method member const

Like in QCryptographicHash, it's never re-set, so, like there, make it immutable.

Pick-to: 6.5
Change-Id: I88f3dc15febffa8950256aedc5e8d1385fc86ddd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/tools/qcryptographichash.cpp

@@ -991,7 +991,7 @@ public:
     QByteArray result;
     QBasicMutex finalizeMutex;
     QCryptographicHash messageHash;
-    QCryptographicHash::Algorithm method;
+    const QCryptographicHash::Algorithm method;
     bool messageHashInited;
 
     void initMessageHash();