The warning message, output during the kernel build, says it all -- WARNING: modpost: missing MODULE_LICENSE() in net/9p/9pnet_xen.o
There is no module 9pnet_xen in upstream kernel. You need to contact the author of any of tree code.
Found it never mind
Thank you for your apology.
Still present in linux-4.15-rc7 as of Mon 8 Jan 2018 (UTC).
Fix submitted (may make it in 4.15 final) and then stable. https://patchwork.ozlabs.org/patch/856908/