[NETFILTER]: ip6_tables: remove redundant structure definitions

Move ip6t_standard/ip6t_error_target/ip6t_error definitions to ip6_tables.h
instead of defining them in each table individually.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>