Cygwin: pty: Revise code to make system_printf() work after close.

- After commit 0365031ce1, the issue
  https://cygwin.com/pipermail/cygwin-patches/2020q2/010259.html
  occurs. This patch fixes the issue.

winsup/cygwin/tty.h

@@ -140,6 +140,8 @@ public:
   void set_master_ctl_closed () {master_pid = -1;}
   static void __stdcall create_master (int);
   static void __stdcall init_session ();
+  void set_switch_to_pcon_out (bool v);
+  void wait_pcon_fwd (void);
   friend class fhandler_pty_common;
   friend class fhandler_pty_master;
   friend class fhandler_pty_slave;