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Currently the licence is indicated via a custom string.  We have SPDX
license identifiers now for this task.

Use SPDX license identifier matching current license string.
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>