Use Q_EMIT instead of emit

In C++20 there are new classes that have member functions named emit().
c.f.:
https://lists.qt-project.org/pipermail/development/2020-February/038812.html
https://en.cppreference.com/w/cpp/io/basic_osyncstream/emit

A similar change was done in the Frameworks:
https://invent.kde.org/frameworks/kio/-/merge_requests/315

src/CheckableSessionModel.cpp

@@ -59,7 +59,7 @@ bool CheckableSessionModel::setData(const QModelIndex &index, const QVariant &va
             _checkedSessions.remove(session);
         }
 
-        emit dataChanged(index, index);
+        Q_EMIT dataChanged(index, index);
         return true;
     }
     return SessionListModel::setData(index, value, role);