liblttng-ctl: Stub out removed variable `error_log_time`

liblttng-ctl: Stub out removed variable `error_log_time`

Observed issue
==============

The ABI diff of liblttng-ctl between lttng-tools 2.13 and lttng-tools
master shows that this variable has been removed.

Cause
=====

The visibility of this symbol was restricted in commit
1944cedb180887d9dfe0906f930df167e714af99.

Solution
========

Add to deprecated symbols file.

Known drawbacks
===============

None.

Change-Id: I553c345c053ab39271871b68f7c2a4d1b1e1d628
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>