ipfw.d: Add required dependency declarations

ipfw.d requires definitions of, at least, in_addr_t and struct ip6_addr,
which it must obtain from CTF info. This is found by looking at
$(sysctl -n kern.bootfile)/kernel, which might not be available in a
jail.

These depends_on declarations ensure that dtrace(1) invocations will
simply abort processing of ipfw.d if the required binaries can't be
found, rather than raising an error.

PR: 283359
MFC after: 1 week