w32: Fix gpgrt_poll for Windows.

* src/estream.c (_gpgrt_poll) [W32]: Do not call the syscall clamp.

The system call clamp may only be used directloy around system calls.
We don't need them here because we use independent threads to
implement non-blocking behaviour. In fact the clamp may lead to a
deadlock.

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