Exclude include of poll.h from NonStop builds - not defined on platform.

Exclude include of poll.h from NonStop builds - not defined on platform.

socket.h has been modified so that poll.h is omitted for OPENSSL_SYS_NONSTOP
builds. The platform configuration is derived from UNIX so the include is
only omitted for NonStop but kept in the OPENSSL_SYS_UNIX include block.

Fixes: #22001
Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22006)