Make working set limit available only on libtorrent 2.0.x builds

You can already control the cache size in libtorrent 1.2.x so it doesn't make sense to implement this limit for all use cases. Also there are some downsides to using working set size to limit memory usage such as unresponsive GUI when limit gets hit.

src/gui/advancedsettings.h

@@ -82,8 +82,10 @@ private:
     // OS dependent settings
 #if defined(Q_OS_WIN)
     QComboBox m_comboBoxOSMemoryPriority;
+#ifdef QBT_USES_LIBTORRENT2
     QSpinBox m_spinBoxMemoryWorkingSetLimit;
 #endif
+#endif
 
 #ifndef Q_OS_MACOS
     QCheckBox m_checkBoxIconsInMenusEnabled;