nf_conntrack: fix -Wunused-const-variable=

When building with W=1, the following warning occurs.

net/netfilter/nf_conntrack_proto_dccp.c:72:27: warning: ‘dccp_state_names’ defined but not used [-Wunused-const-variable=]
 static const char * const dccp_state_names[] = {

We include dccp_state_names in the macro
CONFIG_NF_CONNTRACK_PROCFS, since it is only used in the place
which is included in the macro CONFIG_NF_CONNTRACK_PROCFS.

Fixes: 2bc780499aa3 ("[NETFILTER]: nf_conntrack: add DCCP protocol support")
Signed-off-by: Zhu Wang <wangzhu9@huawei.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Florian Westphal <fw@strlen.de>