libnftables: replace netfilter.h by sanitized header

Signed-off-by: Patrick McHardy <kaber@trash.net>
expr/cmp: fix type size

Since nftables now uses nft_rule_expr_get_u32(), the internal size must
also be a uint32_t.

Fixes complete breakage of any cmp or meta expression.

Signed-off-by: Patrick McHardy <kaber@trash.net>