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.

src/nats.zig

@@ -1,3 +1,17 @@
+// Copyright 2023 torque@epicyclic.dev
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 const std = @import("std");
 
 pub const nats_c = @cImport({

src/win-crosshack.c

@@ -1,3 +1,6 @@
+// These functions are taken from MinGW-w64.
+// In theory, they have been released into the public domain.
+
 #include <winsock2.h>
 #include <ws2tcpip.h>