Cygwin: pty: Revise code waiting for forwarding again.

- After commit 6cc299f0e2, outputs of
  cygwin programs which call both printf() and WriteConsole() are
  frequently distorted. This patch fixes the issue.

winsup/cygwin/fhandler.h

@@ -2218,6 +2218,7 @@ class fhandler_pty_slave: public fhandler_pty_common
   }
   void setup_locale (void);
   void set_freeconsole_on_close (bool val);
+  void wait_pcon_fwd (void);
 };
 
 #define __ptsname(buf, unit) __small_sprintf ((buf), "/dev/pty%d", (unit))