netfilter: fix the warning on unused refcount variable

net/netfilter/nfnetlink_acct.c: In function 'nfnl_acct_try_del':
net/netfilter/nfnetlink_acct.c:329:15: warning: unused variable 'refcount' [-Wunused-variable]
unsigned int refcount;
             ^

Fixes: b54ab92b ("netfilter: refcounter conversions")
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>