netfilter: nft_tproxy: Add missing config check

A config check was missing form the code when using
nf_defrag_ipv6_enable with NFT_TPROXY != n and NF_DEFRAG_IPV6 = n and
this caused the following error:

../net/netfilter/nft_tproxy.c: In function 'nft_tproxy_init':
../net/netfilter/nft_tproxy.c:237:3: error: implicit declaration of function
+'nf_defrag_ipv6_enable' [-Werror=implicit-function-declaration]
   err = nf_defrag_ipv6_enable(ctx->net);

This patch adds a check for NF_TABLES_IPV6 when NF_DEFRAG_IPV6 is
selected by Kconfig.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 4ed8eb65

 ("netfilter: nf_tables: Add native tproxy support")
Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>