nvme-tcp.h: fix SPDX header

For .h files we need to use /* */ style comments.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>

include/linux/nvme-tcp.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * NVMe over Fabrics TCP protocol header.
  * Copyright (c) 2018 Lightbits Labs. All rights reserved.