include: cache a copy of nfnetlink.h

include: cache a copy of nfnetlink.h

If nft is compiled without nftables Linux kernel headers installed, we
hit a compilation error:

src/mnl.c: In function ‘mnl_batch_put’:
src/mnl.c:117:16: error: ‘NFNL_SUBSYS_NFTABLES’ undeclared (first use in
this function)
src/mnl.c:117:16: note: each undeclared identifier is reported only once
for each function it appears in
src/mnl.c: In function ‘mnl_batch_begin’:
src/mnl.c:125:16: error: ‘NFNL_MSG_BATCH_BEGIN’ undeclared (first use in
this function)
src/mnl.c: In function ‘mnl_batch_end’:
src/mnl.c:130:16: error: ‘NFNL_MSG_BATCH_END’ undeclared (first use in
this function)

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>