ipfw: teach ipfw that pfsync is an upper layer protocol

Explicitly add pfsync as a know upper layer protocol so we don't
automatically discard pfsync packets (carried over IPv6).

net.inet6.ip6.fw.deny_unknown_exthdrs defaults to 1, so even if
net.inet.ip.fw.default_to_accept is set to 1 we'd discard pfsync (over
IPv6).

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D40973