apps/s_server.c: Avoid unused variable due to 'no-dtls'
apps/s_server.c: Avoid unused variable due to 'no-dtls'

Fixes #6098

Reviewed-by: Matthias St. Pierre <1910231d24f125aceac7850dc92279eee8d52323@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6104)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl