[NETFILTER]: xt_CONNMARK.c build fix

net/netfilter/xt_CONNMARK.c: In function 'target':
net/netfilter/xt_CONNMARK.c:59: warning: implicit declaration of
function 'nf_conntrack_event_cache'

The warning is due to the following .config:
CONFIG_IP_NF_CONNTRACK=m
CONFIG_IP_NF_CONNTRACK_MARK=y
# CONFIG_IP_NF_CONNTRACK_EVENTS is not set
CONFIG_IP_NF_CONNTRACK_NETLINK=m

This change was introduced by:
http://www.kernel.org/git/?p=linux/kernel/git/davem/net-2.6.19.git;a=commit;h=76e4b41009b8a2e9dd246135cf43c7fe39553aa5

Proposed solution (based on the define in
include/net/netfilter/nf_conntrack_compat.h:

Signed-off-by: Benoit Boissinot <benoit.boissinot@ens-lyon.org>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>