Fix building with wxWidgets 3.1.2

From changelog:
- wx/treebook.h doesn't include wx/treectrl.h (and, via it, wx/textctrl.h) any
  more, include these headers explicitly from your code if necessary.

src/prefs/PrefsDialog.cpp

@@ -34,6 +34,7 @@
 #include <wx/listbook.h>
 
 #include <wx/treebook.h>
+#include <wx/treectrl.h>
 
 #include "../AudioIOBase.h"
 #include "../Prefs.h"