Addressed build failure because of missing #ifdef AF_UNIX guard

CLA: trivial
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4067)

apps/s_client.c

@@ -1522,11 +1522,13 @@ int s_client_main(int argc, char **argv)
         }
     }
 
+#ifdef AF_UNIX
     if (socket_family == AF_UNIX && socket_type != SOCK_STREAM) {
         BIO_printf(bio_err,
                    "Can't use unix sockets and datagrams together\n");
         goto end;
     }
+#endif
 
 #ifndef OPENSSL_NO_SCTP
     if (protocol == IPPROTO_SCTP) {