kernel: netfilter: Add nf_defrag_ipv6.ko to NF_CONNTRACK on 4.19

kernel: netfilter: Add nf_defrag_ipv6.ko to NF_CONNTRACK on 4.19

In kernel commit a0ae2562c6c ("netfilter: conntrack: remove l3proto
abstraction") The modules nf_conntrack_ipv4.ko, nf_conntrack_ipv6.ko and
nf_conntrack.ko were squashed together into one module. This module now
depends on nf_defrag_ipv6 when IPv6 support was activated. This is part
of the main netfilter packages, so add nf_defrag_ipv6.ko also to the
default netfilter packages on kernel 4.19 and later.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>