extensions/libxt_tcp: fix nftables translate flags value, 'none' vs '0x0'

extensions/libxt_tcp: fix nftables translate flags value, 'none' vs '0x0'

The iptables command:
 -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE

should translate to:
 tcp flags & fin|syn|rst|psh|ack|urg == 0x0

instead of:
 tcp flags & fin|syn|rst|psh|ack|urg == none

Reported-by: Vadim A. Misbakh-Soloviov <netfilter@mva.name>
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Tested-by: Vadim A. Misbakh-Soloviov <netfilter@mva.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>