Add missing EPOCH(9) wrapper in ipfw(8).

Backtrace:
panic()
ip_output()
dyn_tick()
softclock_call_cc()
softclock()
ithread_loop()

Differential Revision: https://reviews.freebsd.org/D23599
Reviewed by: glebius@ and ae@
Found by: mmacy@
Reported by: jmd@
Sponsored by: Mellanox Technologies