Wrap use of poll.h to prevent including on NonStop.

Wrap use of poll.h to prevent including on NonStop.

Fixes: #26724
Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/26726)

(cherry picked from commit ff030ad5bd1c6196e640b1338dac23c1ce3a3154)