Adding AUTHORS and changing copyright headers to refer to it.

Change-Id: Id3306370911baf01bd2137dc0fbc8c421682e9f7
diff --git a/src/mpool.c b/src/mpool.c
index 6b5824e..b313e85 100644
--- a/src/mpool.c
+++ b/src/mpool.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 2018 Google LLC
+ * Copyright 2018 The Hafnium Authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.