bt2: compile `native_bt.c` with `-Wno-undef`

bt2: compile `native_bt.c` with `-Wno-undef`

As explained in the comment, work around a problem with SWIG 4.2.0.

Change-Id: Ia95fc1e4929591a0a16df48892b05f5e5df46be3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11849
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11930
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>