pf: always drop ICMPv6 in IPv4 datagrams

Not only when compiled with INET6.

Suggested by Max Laier.
ok henning@

Reviewed by: zlei
Obtained from: OpenBSD, jsing <jsing@openbsd.org>, 0ba1e6e32b9
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46575