ipdbg-la: Fix data_available() implementation on Windows.

ipdbg-la: Fix data_available() implementation on Windows.

  [...]/protocol.c: In function 'data_available':
  [...]/protocol.c:73:38: error: 'bytes_available' undeclared (first use in this function)
    ioctlsocket(tcp->socket, FIONREAD, &bytes_available);
                                        ^
  [...]/protocol.c:73:38: note: each undeclared identifier is reported only once for each function it appears in
  [...]/protocol.c:84:1: warning: no return statement in function returning non-void [-Wreturn-type]
   }
   ^