[contrib][linux] Fix up SPDX license identifiers

Correctly identify that we are GPL v2+ or BSD 3 clause, as pointed out
in issue #2663.

contrib/linux-kernel/linux_zstd.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
+/* SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause */
 /*
  * Copyright (c) Yann Collet, Facebook, Inc.
  * All rights reserved.