Fix linking of www-servers/nginx with LibreSSL 2.1.2

Related discussion / patch:
* https://www.mail-archive.com/tech@openbsd.org/msg21480.html
* http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/www/nginx/patches/Attic/patch-src_event_ngx_event_openssl_h?rev=1.2