tcp_metrics: tcpm_hash_bucket is strictly local

After commit 098a697b49 ("tcp_metrics: Use a single hash table
for all network namespaces."), tcpm_hash_bucket is local to
net/ipv4/tcp_metrics.c

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/ipv4.h

@@ -11,7 +11,6 @@
 #include <linux/rcupdate.h>
 #include <linux/siphash.h>
 
-struct tcpm_hash_bucket;
 struct ctl_table_header;
 struct ipv4_devconf;
 struct fib_rules_ops;