Fix compilation on Windows (Remove using namespace libtorrent; from headers)

src/preferences/options_imp.cpp

@@ -50,6 +50,8 @@
 #include "scannedfoldersmodel.h"
 #include "qinisettings.h"
 
+using namespace libtorrent;
+
 // Constructor
 options_imp::options_imp(QWidget *parent):QDialog(parent){
   qDebug("-> Constructing Options");

src/preferences/options_imp.h

@@ -37,8 +37,6 @@
 // actions on double-click on torrents
 enum DoubleClickAction {TOGGLE_PAUSE, OPEN_DEST, NO_ACTION};
 
-using namespace libtorrent;
-
 class QCloseEvent;
 class AdvancedSettings;