Change to lt namespace

Quoting from libtorrent doc:
> In the future, libtorrent will be the alias and lt the namespace name.

src/base/bittorrent/private/portforwarderimpl.h

@@ -60,6 +60,6 @@ private:
     void stop();
 
     bool m_active;
-    libtorrent::session *m_provider;
+    lt::session *m_provider;
     QHash<quint16, std::vector<LTPortMapping>> m_mappedPorts;
 };