uapi: add SPDX identifier to vm_sockets_diag.h

New file seems to have missed the SPDX license scan and update.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>