nft: fix inconsistent data type in NFT_EXPR_CMP_OP and NFT_EXPR_META_KEY

We have to use uint32_t instead uint8_t to adapt this to the libnftables
changes.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
nft: Use new libnftnl library name against former libnftables

Adapt the current code to use the new library name libnftnl.

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