cpp-common: rename bt2c::makeUnique() -> bt2s::make_unique()

cpp-common: rename bt2c::makeUnique() -> bt2s::make_unique()

I now want everything mimicking the STL stuff to have the same names,
but I don't want to mix coding styles in `bt2c`.

Therefore introduce this new `bt2s` (the `s` stands for `std`) namespace
to contain everything acting as the STL (or Boost, if need be).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ie24c11170cc650028f3ce50fcab2004f0195c12f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11393
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>