Make it compile against qt5.15

src/kmanagesieve/sessionthread.cpp

@@ -84,7 +84,11 @@ void SessionThread::doInit()
     Q_ASSERT(QThread::currentThread() == thread());
     m_socket = std::make_unique<QSslSocket>();
     connect(m_socket.get(), &QSslSocket::readyRead, this, &SessionThread::slotDataReceived);
+#if QT_VERSION < QT_VERSION_CHECK(5, 15, 0)
     connect(m_socket.get(), QOverload<QAbstractSocket::SocketError>::of(&QAbstractSocket::error), this, &SessionThread::slotSocketError);
+#else
+    connect(m_socket.get(), QOverload<QAbstractSocket::SocketError>::of(&QAbstractSocket::errorOccurred), this, &SessionThread::slotSocketError);
+#endif
     connect(m_socket.get(), &QSslSocket::disconnected, this, &SessionThread::socketDisconnected);
     connect(m_socket.get(), &QSslSocket::connected, this, &SessionThread::socketConnected);
 }