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/20231127190319.1190813-1-sdf@google.com/

Fixes: 11614723 ("xsk: Add option to calculate TX checksum in SW")
Suggested-by: Simon Horman <horms@kernel.org>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231204174231.3457705-1-sdf@google.com

Documentation/networking/xsk-tx-metadata.rst

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