xsk: Add missing SPDX to AF_XDP TX metadata documentation

Not sure how I missed that. I even acknowledged it explicitly
in the changelog [0]. Add the tag for real now.

[0] https://lore.kernel.org/bpf/[e-mail]/

Fixes: 11614723af26 ("xsk: Add option to calculate TX checksum in SW")
Suggested-by: Simon Horman <[e-mail]>
Signed-off-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/networking/xsk-tx-metadata.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
 ==================
 AF_XDP TX Metadata
 ==================