[NETFILTER]: Fix non-ANSI func. decl.

Fix non-ANSI function declaration:
net/netfilter/nf_conntrack_core.c:1096:25: warning: non-ANSI function declaration of function 'nf_conntrack_flush'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>