X25: constify null_x25_address

null_x25_address is only used to access the string it contains, so it can
be const.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/x25/af_x25.c

@@ -74,7 +74,7 @@ DEFINE_RWLOCK(x25_list_lock);
 
 static const struct proto_ops x25_proto_ops;
 
-static struct x25_address null_x25_address = {"               "};
+static const struct x25_address null_x25_address = {"               "};
 
 #ifdef CONFIG_COMPAT
 struct compat_x25_subscrip_struct {