lttng-ctl: Hide symbol introduced by fix

lttng-ctl: Hide symbol introduced by fix

Michael Jeanson noticed that f94318611 mistakenly introduces a new
public symbol in liblttng-ctl.so. This change was not intended and is
due to a bad backport of a fix initially developed against the master
branch.

The master branch (and upcoming version) of LTTng-tools is built with
the -fvisibility=hidden. Hence, the initial version of the fix had no
need to hide the utils_create_lock_file symbol.

Since the supported stable releases (2.12 and 2.13) are not built with
those options, that symbol has to be explicitly marked as hidden.

Reported-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I874538317617003eb1c58c2d3b7b0a2bdf905ef7