add missing stdlib import

wx_Treebook uses std::function which is not defined (at least) when
using wxWidgets 3.0.5.1. Fix this by adding the import

include/widgets/wx_treebook.h

@@ -24,6 +24,7 @@
 #ifndef WX_TREEBOOK_H
 #define WX_TREEBOOK_H
 
+#include <functional>
 #include <wx/treebook.h>
 
 class WX_TREEBOOK : public wxTreebook