uapi: add SPDX identifier to vm_sockets_diag.h

New file seems to have missed the SPDX license scan and update.
Signed-off-by: NStephen Hemminger <sthemmin@microsoft.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/vm_sockets_diag.h

+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
 /* AF_VSOCK sock_diag(7) interface for querying open sockets */
 
 #ifndef _UAPI__VM_SOCKETS_DIAG_H__