core,w32: Use timeout in es_poll even if there are no FDs.

* src/w32-estream.c (_gpgrt_w32_poll): Sleep if there are no handles
to wait for.

This better matches the semantics of poll and select and avoid busy
waitins. We had this problem for example in the LDAP helper process
reaper of GnuPG's Dirmngr.

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