Add missing licence notice to the cache module (#2138)

vulkano/src/cache.rs

@@ -1,3 +1,12 @@
+// Copyright (c) 2016 The vulkano developers
+// 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. All files in the project carrying such
+// notice may not be copied, modified, or distributed except
+// according to those terms.
+
 use ahash::HashMap;
 use parking_lot::RwLock;
 use std::{collections::hash_map::Entry, hash::Hash};