Add missing license

Author: ldionne

libcxx/src/experimental/keep.cpp

@@ -1,3 +1,10 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
 
 // Empty file just to make sure we produce a libc++experimental.a library even
 // when there isn't anything in it.