Make no-psk compile without warnings.

PR#4035

Reviewed-by: Matt Caswell <matt@openssl.org>
diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index 7571095..1caa3c8 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -333,13 +333,13 @@
         goto err;
     if (!custom_exts_copy(&ret->srv_ext, &cert->srv_ext))
         goto err;
-
+#ifndef OPENSSL_NO_PSK
     if (cert->psk_identity_hint) {
         ret->psk_identity_hint = BUF_strdup(cert->psk_identity_hint);
         if (ret->psk_identity_hint == NULL)
             goto err;
     }
-
+#endif
     return (ret);
 
  err: