Fix: bt2: use unsigned iteration variable

Fix: bt2: use unsigned iteration variable

This fixes the following compiler diagnostic (gcc 9.1.0):

    bt2/native_bt.c: In function ‘_wrap_field_class_enumeration_unsigned_get_mapping_labels_for_value’:
    bt2/native_bt.c:9387:25: error: comparison of integer expressions of different signedness: ‘int’ and ‘uint64_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]
     9387 |       for (int i = 0; i < *arg4; i++) {
          |                         ^

Change-Id: I543c61d64c0cf7b42cb242bdbb7b71cbf67dbe48
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1834
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>