lib: internal: object-pool.h: require logging

lib: internal: object-pool.h: require logging

This patch makes the internal `object-pool.h` require what it needs
instead of using the macros only if they exist. This makes it impossible
to skip logging in this header.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I5f1b5550dfca7c7f3e8d62134a2917b059e4da0c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1580
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>