Use typeof instead of __auto_type

Use typeof instead of __auto_type

We need to support gcc 4.8, which doesn't support __auto_type.  We can
use typeof instead.

Change-Id: I29c58b0270eef3b841dfb8d24c704e87ff756d8d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2256
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>