Exclude header demonstration in propgrid sample when wxUSE_HEADERCTRL == 0.

Code responsible for header demonstration should be excluded in this case because header management functions in wxPropertyGridManager depend on wxHeaderCtrl (via wxPGHeaderCtrl) and if it is not present then respective methods in wxPG are not exposed.

samples/propgrid/propgrid.h

@@ -191,7 +191,9 @@ public:
     void OnClearModifyStatusClick( wxCommandEvent& event );
     void OnFreezeClick( wxCommandEvent& event );
     void OnEnableLabelEditing( wxCommandEvent& event );
+#if wxUSE_HEADERCTRL
     void OnShowHeader( wxCommandEvent& event );
+#endif
     void OnDumpList( wxCommandEvent& event );
     void OnCatColours( wxCommandEvent& event );
     void OnSetColumns( wxCommandEvent& event );