lib: add logging format specifier for bt_field_location

lib: add logging format specifier for bt_field_location

Add the `format_field_location` function, to format a bt_field_location
object in logging messages.  Add the format specifier `L`, to format
bt_field_location objects in lib-logging format strings.

Change-Id: Ia874bea6a110e051cc19a0250e5cd831888f849f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7317
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7296