src: fix compilation due to missing NFPROTO_INET definition

This adds a copy of the include/linux/netfilter.h kernel header
that defines NFPROTO_INET, so libnftables compiles with relatively
old kernel headers in the system.

chain.c: In function 'nft_hooknum2str':
chain.c:53:7: error: 'NFPROTO_INET' undeclared (first use in this function)

This required to reorder and remove unneeded headers in src/expr/data_reg.c
to avoid a compilation warning due to redefinition of __visible.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>