cpp-common/bt2: add `noexcept` to user attribute setters

cpp-common/bt2: add `noexcept` to user attribute setters

The bt_*_set_user_attributes() functions never fail (they only increment
the reference count of the received value), therefore the user attribute
setters never throw.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I5b9c05e6efd7b42965b9e075438b518df1a5da90
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11229
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>