netfilter: fix fallout from xt/nf osf separation

Stephen Rothwell says:
  today's linux-next build (x86_64 allmodconfig) produced this warning:
  ./usr/include/linux/netfilter/nf_osf.h:25: found __[us]{8,16,32,64} type without #include <linux/types.h>

Fix that up and also move kernel-private struct out of uapi (it was not
exposed in any released kernel version).

tested via allmodconfig build + make headers_check.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: bfb15f2a ("netfilter: extract Passive OS fingerprint infrastructure from xt_osf")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>