Don't rely on wx/button.h being included from wx/*picker.h

Include this header explicitly in the sample instead of relying on its
indirect inclusion from wx/filepicker.h or wx/dirpicker.h respectively.

No changes yet, but this will become important once wx/gtk/filepicker.h
doesn't include wx/button.h any longer.

samples/widgets/dirpicker.cpp

@@ -25,6 +25,7 @@
 // for all others, include the necessary headers
 #ifndef WX_PRECOMP
     #include "wx/app.h"
+    #include "wx/button.h"
     #include "wx/log.h"
     #include "wx/radiobox.h"
     #include "wx/statbox.h"