net: netconsole: Constify struct config_item_type

net: netconsole: Constify struct config_item_type

'struct config_item_type' is not modified in this driver.

This structure is only used with config_group_init_type_name() which takes
a const struct config_item_type* as a 3rd argument.

This also makes things consistent with 'netconsole_target_type' witch is
already const.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  33007	   3952	   1312	  38271	   957f	drivers/net/netconsole.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  33071	   3888	   1312	  38271	   957f	drivers/net/netconsole.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/9c205b2b4bdb09fc9e9d2cb2f2936ec053da1b1b.1723325900.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>