Fix compilation warning with SerializeClientConnectionInfo()

Fix compilation warning with SerializeClientConnectionInfo()

This function uses an argument named "maxsize" that is only used in
assertions, being set once outside the assertion area.  Recent gcc
versions with -Wunused-but-set-parameter complain about a warning when
building without assertions enabled, because of that.

In order to fix this issue, PG_USED_FOR_ASSERTS_ONLY is added to the
function argument of SerializeClientConnectionInfo(), which is the first
time we are doing so in the tree.  The CI is fine with the change, but
let's see what the buildfarm has to say on the matter.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Jacob Champion <jchampion@postgresql.org>
Discussion: https://postgr.es/m/pevajesswhxafjkivoq3yvwxga77tbncghlf3gq5fvchsvfuda@6uivg25sb3nx
Backpatch-through: 16