inpcb: Move the definition of struct inpcblbgroup to in_pcb_var.h

It's only needed for in_pcb.c and in6_pcb.c, so can go to the private
header.

No functional change intended.

Reported by: glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield

(cherry picked from commit ca94f92c23fd09b28ac3398657ae2ae9367bcdf5)