CLEANUP: ssl: silence a build warning when threads are disabled

CLEANUP: ssl: silence a build warning when threads are disabled

Building without threads now shows this warning:

src/ssl_sock.c: In function 'cli_io_handler_commit_cert':
src/ssl_sock.c:12121:24: warning: unused variable 'bind_conf' [-Wunused-variable]
      struct bind_conf *bind_conf = ckchi->bind_conf;
                        ^~~~~~~~~

This is because the variable is needed only to unlock the structure, and
the unlock operation does nothing in this case. Let's mark the variable
__maybe_unused for this, but it would be convenient in the long term if
we could make the thread macros pretend they consume the argument so that
this remains less visible outside.

No backport is needed.