Fix nooptions VIMAGE build

The recent FIBify commits deref struct thread without including
sys/proc.h, which can result in a compiler error. This becomes
apparent when building with LINT-NOVIMAGE, as net/vnet.h includes
sys/proc.h. Fix this by directly including sys/proc.h

Fixes: 4c486fe40267 ("ip_mroute: FIBify"), 0bb9c2b665d9 ("ip6_mroute: FIBify")