Cygwin: pty: Use system NLS function instead of PTY's own one.

- Since calling system __loadlocale() caused execution error,
  PTY used its own NLS function. The cause of the error has been
  found, the corresponding code has been rewritten using system
  function.

winsup/cygwin/fhandler.h

@@ -2192,6 +2192,7 @@ class fhandler_pty_slave: public fhandler_pty_common
   {
     return get_ttyp ()->ti.c_lflag & ICANON;
   }
+  void setup_locale (void);
 };
 
 #define __ptsname(buf, unit) __small_sprintf ((buf), "/dev/pty%d", (unit))