Add license header

Author: vks

rand_core/src/mock.rs

@@ -1,3 +1,11 @@
+// Copyright 2021 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
 use crate::{impls, Error, RngCore};
 
 /// A simple implementation of `RngCore` for testing purposes.