Move `src/plugins/comp-logging.h` -> `src/logging/comp-logging.h`

Move `src/plugins/comp-logging.h` -> `src/logging/comp-logging.h`

Move this file so that other parts of the project than plugins, namely
Python bindings, can use it.

Also: in `comp-logging.h`, the requirement that `BT_LOG_OUTPUT_LEVEL`
and `BT_COMP_LOG_SELF_COMP` are set is removed because they are not
needed to use the generic BT_COMP_LOG_CUR_LVL() macro.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic961c3a97863dcef63fb7ee673841650cf9ca9e9
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1788
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>