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/tty.h

@@ -107,6 +107,7 @@ private:
   UINT term_code_page;
   bool need_redraw_screen;
   HANDLE fwd_done;
+  DWORD pcon_last_time;
 
 public:
   HANDLE from_master () const { return _from_master; }