sys/netpfil/pf: fix non-INET module build

pf.ko, when built as a module without 'options INET' but with 'options
VIMAGE', won't load:

link_elf_obj: symbol vnet_entry_in_loopback_mask undefined

This is because it uses IN_LOOPBACK(), which in the VIMAGE case uses
INET-specific symbols.

Fix by making this check conditional on #ifdef INET.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1157