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/filterparserthread.h

@@ -43,7 +43,7 @@ public:
     FilterParserThread(QObject *parent = nullptr);
     ~FilterParserThread();
     void processFilterFile(const QString &filePath);
-    libtorrent::ip_filter IPfilter();
+    lt::ip_filter IPfilter();
 
 signals:
     void IPFilterParsed(int ruleCount);
@@ -62,7 +62,7 @@ private:
 
     bool m_abort;
     QString m_filePath;
-    libtorrent::ip_filter m_filter;
+    lt::ip_filter m_filter;
 };
 
 #endif // BITTORRENT_FILTERPARSERTHREAD_H

src/base/bittorrent/private/portforwarderimpl.cpp

@@ -37,7 +37,7 @@
 
 const QString KEY_ENABLED = QStringLiteral("Network/PortForwardingEnabled");
 
-PortForwarderImpl::PortForwarderImpl(libtorrent::session *provider, QObject *parent)
+PortForwarderImpl::PortForwarderImpl(lt::session *provider, QObject *parent)
     : Net::PortForwarder {parent}
     , m_active {SettingsStorage::instance()->loadValue(KEY_ENABLED, true).toBool()}
     , m_provider {provider}

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;
 };