netfilter: xtables: disable 32bit compat interface by default

This defaulted to 'y' because before this knob existed the 32bit
compat layer was always compiled in if CONFIG_COMPAT was set.

32bit iptables on 64bit kernel isn't common anymore, so remove
the default-y now.

Signed-off-by: Florian Westphal <fw@strlen.de>