w32: Use CancelSynchronousIo in destroy_reader.

* src/w32-util.c (_gpgme_w32_cancel_synchronous_io): New.
* src/w32-io.c (destroy_reader): Use it here.

This has not been tested but should on Vista and later help to fix a
possible hang.

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