tcp: cdg: make struct tcp_cdg static

The structure tcp_cdg is local to the source and
does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'tcp_cdg' was not declared. Should it be static?
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_cdg.c

@@ -389,7 +389,7 @@ static void tcp_cdg_release(struct sock *sk)
 	kfree(ca->gradients);
 }
 
-struct tcp_congestion_ops tcp_cdg __read_mostly = {
+static struct tcp_congestion_ops tcp_cdg __read_mostly = {
 	.cong_avoid = tcp_cdg_cong_avoid,
 	.cwnd_event = tcp_cdg_cwnd_event,
 	.pkts_acked = tcp_cdg_acked,