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

lib/libnv/Makefile

@@ -9,7 +9,7 @@ LIB=	nv
 SHLIB_MAJOR= 0
 
 .PATH: ${SRCTOP}/sys/contrib/libnv ${SRCTOP}/sys/sys
-CFLAGS+=-I${SRCTOP}/sys -I${.CURDIR}
+CFLAGS+=-I${.CURDIR}
 
 SRCS=	cnvlist.c
 SRCS+=	dnvlist.c