Make wx/buffer.h header self-contained

Including this header as the first wxWidgets header in a translation
unit resulted in compilation errors due to including the internal
wx/wxcrtbase.h header before including wx/defs.h, which must be included
before it (at least to define DLL import/export macros and sized char
types used in wx/wxcrtbase.h).

Fix this by simply including wx/defs.h from wx/buffer.h.

include/wx/buffer.h

@@ -11,7 +11,7 @@
 #ifndef _WX_BUFFER_H
 #define _WX_BUFFER_H
 
-#include "wx/chartype.h"
+#include "wx/defs.h"
 #include "wx/wxcrtbase.h"
 
 #include <stdlib.h>             // malloc() and free()