Use <stddef.h> to get wchar_t, rather than <wchar.h> (or nothing, in putty.h).

Both are required to contain wchar_t in C99, but only <stddef.h> does in the
version of MPW I've got here.

[originally from svn r2206]

wcwidth.c

@@ -6,7 +6,7 @@
  * Markus Kuhn -- 2001-01-12 -- public domain
  */
 
-#include <wchar.h>
+#include <stddef.h>
 
 struct interval {
   unsigned short first;