estream,w32: Temporary fix for gpgrt_poll.

* src/estream.c (_gpgrt_poll) [W32]: Do not use FD_ISSET.

gpgrt_poll return an error on Windows anyway and thus it does not make
sense to use a possible undefined macro here.

• Reported-by: Andre Heinecke
• Signed-off-by: Werner Koch <wk@gnupg.org>