Fix warning: value computed is not used [-Wunused-value]

In file included from net/ipv6/netfilter/ip6t_LOG.c:26:0:
include/net/netfilter/xt_log.h: In function 'sb_close':
include/net/netfilter/xt_log.h:50:3: warning: value computed is not used [-Wunused-value]

Change-Id: I51fad10f314fafd9134ca0a8d24c9e61240178d5