cli: Rename `logging.c` to `logging.cpp` to force C++ linking

This is to force the linking of the babeltrace2 binary using a C++
linker. This is necessary to support the Babeltrace2 static build
once we start depending on C++ symbols.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ife9967d9362c7648b6ea031f453262a10aa03483
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7954
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9597
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: Simon Marchi <simon.marchi@efficios.com>