Fix to expose a GUC variable, Log_disconnections, to outside of postgres.c

Hi,

I just found that Log_disconnections value has not been
exposed to outside of postgres.c.
Despite that, Log_connections has already been exposed.

So, I'd like to add attached fix to touch the Log_disconnections
value in other modules.

Any comments?

Regards,
--
NAGAYASU Satoshi <snaga@uptime.jp>