allow builds with CONFIG_NET disabled

[ddiss]

arch/lkl/Kconfig currently carries:

 17        select NET
 18        select MULTIUSER
 19        select INET
 20        select IPV6

This keeps things relatively simple in that tools/lkl/lib/net.c etc. can be built unconditionally. Still, there's no particular reason to only support lkl builds alongside CONFIG_NET, given that tools such as lklfuse have no specific need for anything network related.