Add license info

It's easy to pick the apache license because that's what nats.c uses.
It's a permissive license, and there's no need to get fancy here.

build.zig

@@ -1,3 +1,6 @@
+// This file is licensed under the CC0 1.0 license.
+// See: https://creativecommons.org/publicdomain/zero/1.0/legalcode
+
 const std = @import("std");
 const nats_build = @import("./nats-c.build.zig");