Include poll.h instead of incorrect sys/poll.h

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Anton Arapov <anton@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21131)

include/internal/sockets.h

@@ -114,7 +114,7 @@ struct servent *PASCAL getservbyname(const char *, const char *);
 #  endif
 
 #  ifdef OPENSSL_SYS_UNIX
-#    include <sys/poll.h>
+#    include <poll.h>
 #    include <errno.h>
 #  endif