netfilter: xt_LOG: add missing string format in nf_log_packet()

   net/netfilter/xt_LOG.c: In function 'log_tg':
>> net/netfilter/xt_LOG.c:43: error: format not a string literal and no format arguments

Fixes: fab4085f ("netfilter: log: nf_log_packet() as real unified interface")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>