netfilter: fix warning about invalid const usage

This patch fixes the declaration of the logger structure in ebt_log
and ebt_ulog: I forgot to remove the const option from their declaration
in the commit ca735b3a ("netfilter:
use a linked list of loggers").

Pointed-out-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>