netfilter: nft_tproxy: Fix missing-braces warning

This patch fixes a warning reported by the kbuild test robot (from linux-next
tree):
   net/netfilter/nft_tproxy.c: In function 'nft_tproxy_eval_v6':
>> net/netfilter/nft_tproxy.c:85:9: warning: missing braces around initializer [-Wmissing-braces]
     struct in6_addr taddr = {0};
            ^
   net/netfilter/nft_tproxy.c:85:9: warning: (near initialization for 'taddr.in6_u') [-Wmissing-braces]

This warning is actually caused by a gcc bug already resolved in newer
versions (kbuild used 4.9) so this kind of initialization is omitted and
memset is used instead.

Fixes: 4ed8eb65

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