netfilter: xt_hashlimit: fix unused variable warning if IPv6 disabled

Fixes this warning when CONFIG_IP6_NF_IPTABLES is not enabled:

net/netfilter/xt_hashlimit.c: In function ‘hashlimit_init_dst’:
net/netfilter/xt_hashlimit.c:448:9: warning: unused variable ‘frag_off’ [-Wunused-variable]

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>