configure: enable -Wshadow-field

This warning exists in clang and warns about fields in classes shadowing
fields in base classes.  I hit one bug that this warning would have
caught.  I don't think there are many legitimate / useful cases to have
derived and base classes with fields of the same name.

Change-Id: Ifab0bc03bce2b84a4a186fcc98da8b68ef61e771
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8514
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>