seccomp: allow openat()

It seems some libc uses openat() directly when open()
is called.

Resolves: #185

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>

src/worker-privs.c

@@ -121,6 +121,7 @@ int disable_system_calls(struct worker_st *ws)
 		if (vhost->perm_config.config->xml_config_file) {
 			ADD_SYSCALL(stat, 0);
 			ADD_SYSCALL(open, 0);
+			ADD_SYSCALL(openat, 0);
 			break;
 		}
 	}