libnv: Remove -I${SRCTOP}/sys

This should have been done as part of r336019 -- including ${SRCTOP}/sys is
not a good business model for something that's build in legacy/bootstrap
stages.

Beyond that, libnv seems to build quite alright as legacy, part of
buildworld, and standalone without. Axe it.

Reported by: truckman (head building stable/11)
Tested by: Shawn Webb (HardenedBSD)
MFC after: 3 days