Fix linking to wxObjectEventFunctor with Clang on Windows

Linking specific applications (sockets/baseserver, base test) resulted in the
following error: undefined reference to `__imp__ZTV20wxObjectEventFunctor'
Fix this by implementing the destructor inside the base library.

include/wx/event.h

@@ -213,6 +213,8 @@ private:
 class WXDLLIMPEXP_BASE wxObjectEventFunctor : public wxEventFunctor
 {
 public:
+    virtual ~wxObjectEventFunctor();
+
     wxObjectEventFunctor(wxObjectEventFunction method, wxEvtHandler *handler)
         : m_handler( handler ), m_method( method )
         { }