MINOR: log: allow the HTTP status code to be logged even in TCP frontends

MINOR: log: allow the HTTP status code to be logged even in TCP frontends

Log format is defined in the frontend, and some frontends may be chained to
an HTTP backend. Sometimes it's very convenient to be able to log the HTTP
status code of these HTTP backends. This status is definitely present in
the internal structures, it's just that we used to limit it to be used in
HTTP frontends. So let's simply relax the check to allow it to be used in
TCP frontends as well.