build: avoid symbol namespace pollution

build: avoid symbol namespace pollution

As of f40eabb01 (add pkt_buff and protocol helper functions)
libnetfilter_queue accidentally exports the internal function named
'checksum'.  This is a bit too generic and may cause crashes with
applications that worked fine before.

This patch makes the functions checksum, checksum_tcpudp_ipv4 and
checksum_tcpudp_ipv6 local by building with fvis-hidden and adding
EXPORTs for the legacy api calls and the ones that seem to have missing
EXPORT tags (mainly pktbuff api).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>