Avoid mixing const and non-const iterator

Fix the HandlersRegistry::findHandler function, avoid mixing const and
non-const iterator.

Change-Id: I81d6c1495850ad230db26c5f7af606ceca90a0d5
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/protobuf/qprotobufserializer.cpp

@@ -39,7 +39,7 @@ struct HandlersRegistry
         QtProtobufPrivate::SerializationHandler handler;
         QReadLocker locker(&m_lock);
         auto it = m_registry.constFind(type);
-        if (it != m_registry.end())
+        if (it != m_registry.constEnd())
             handler = it.value();
         return handler;
     }