Addressed build failure because of missing #ifdef AF_UNIX guard
CLA: trivial

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4067)
diff --git a/apps/s_client.c b/apps/s_client.c
index 45464a4..5ae52b4 100644
--- a/apps/s_client.c
+++ b/apps/s_client.c
@@ -1522,11 +1522,13 @@
         }
     }
 
+#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) {