netfilter: ebtables: replace zero-length array members

As suggested by Kees[1], replace the old-style 0-element array members
of multiple structs in ebtables.h with modern C99 flexible array.

[1]: https://lore.kernel.org/all/5E8E0F9C-EE3F-4B0D-B827-DC47397E2A4A@kernel.org/

[ fw@strlen.de:
  keep struct ebt_entry_target as-is, causes compiler warning:
  "variable sized type 'struct ebt_entry_target' not at the end of a
  struct or class is a GNU extension" ]

Link: https://github.com/KSPP/linux/issues/21


Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Florian Westphal <fw@strlen.de>