Add short license and copyright notice

lib/std/io_uring.zig

@@ -1,3 +1,8 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2020 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
 const builtin = @import("builtin");
 const std = @import("std");
 const assert = std.debug.assert;