r381568 added CONFIGURE_ARGS+=--with-out-mib-modules="mibII/ipv6", but the handling of other options can result in two different --with-out-mib-modules arguments being passed to configure, and only the second one has effect. That leads to build failures because it tries to build the old mibII/ipv6 which is no longer supported on freebsd.
This change adds mibII/ipv6 to the list of excluded modules that gets turned into the --with-out-mib-modules argument.