Use in6addr_loopback instead of IN6ADDR_LOOPBACK_INIT

The latter is unavailable on OpenBSD with _POSIX_SOURCE defined

main.c

@@ -165,7 +165,7 @@ int main(int argc, char *argv[])
 	const struct sockaddr_in6 haddr = {
 		.sin6_family = AF_INET6,
 		.sin6_port = htons(PORT),
-		.sin6_addr = IN6ADDR_LOOPBACK_INIT,
+		.sin6_addr = in6addr_loopback,
 	};
 	if (bind(sfd, (const struct sockaddr *)&haddr, sizeof(haddr)) == -1) {
 		fprintf(stderr, "Error binding socket to address\n");