Flush winpty-agent --show-input output

This is useful when winpty-agent is invoked from the Win32-OpenSSH server,
which uses a proper console for STDIN, but sockets(!) for STDOUT and
STDERR.  The C++ runtime uses block buffering on the socket output, so
flush explicitly.

src/agent/DebugShowInput.cc

@@ -200,6 +200,7 @@ void debugShowInput(bool enableMouse) {
             }
         }
         std::cout << ss.str();
+        std::cout.flush();
     }
     SetConsoleMode(conin, origConsoleMode);
 }