include: net: Remove repeated struct declaration

struct ctl_table_header is declared twice. One is declared
at 46th line. The blew one is not needed. Remove the duplicate.
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -407,7 +407,6 @@ int register_pernet_device(struct pernet_operations *);
 void unregister_pernet_device(struct pernet_operations *);
 
 struct ctl_table;
-struct ctl_table_header;
 
 #ifdef CONFIG_SYSCTL
 int net_sysctl_init(void);