Add license header to impls.rs

Signed-off-by: Nico Burns <[email protected]>

malloc_size_of/src/impls.rs

@@ -1,3 +1,13 @@
+// Copyright 2016-2017 The Servo Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
 use crate::{MallocShallowSizeOf, MallocSizeOf, MallocSizeOfOps};
 use core::cell::{Cell, RefCell};