src/cpp-common: add bt2_common::makeUnique()

src/cpp-common: add bt2_common::makeUnique()

This is our equivalent of std::make_unique() (introduced in C++14) for
C++11.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic58341f7419ac447f83401358ca0884151246dc8
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7465
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10815
Tested-by: jenkins <jenkins@lttng.org>