netfilter: nf_defrag_ipv6.o included twice

'nf_defrag_ipv6' is built as a separate module; it shouldn't be
included in the 'nf_conntrack_ipv6' module as well.

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>