libnftables: fix header export

libnftables: fix header export

Instruct Make to actually install the header to the system, otherwise
users won't see the header in their system after running 'make install'.

Also, export main libnftables header with a proper name, since we have another
private header called 'nftables.h' (i.e, let's be concrete with the naming).

Signed-off-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>