liblttng-ctl: Hide mangled C++ symbols

liblttng-ctl: Hide mangled C++ symbols

A number of functions in C++ have default visibility. To avoid those
symbols being erroneously available to those who load the liblttng-ctl
library, a version script has been added to set the visibility of all
name-mangled symbols to local.

Change-Id: If2df7cfbadb0d1b9d3747cdf4990cbc942118e6f
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>