Fix: sessiond: error accounting: wrong sizeof argument cpu_counters

Fix: sessiond: error accounting: wrong sizeof argument cpu_counters

`cpu_counters` is an array of `struct lttng_ust_abi_object_data *`, not
of `struct lttng_ust_abi_object_data`. This didn't cause any problems
since the structure is larger than a pointer.

1452203 Wrong sizeof argument

The wrong sizeof value is used in an expression or as argument to a function. The result is an incorrect value that may cause unexpected program behaviors.

In event_notifier_error_accounting_register_app: The sizeof operator is invoked on the wrong argument (CWE-569)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9278949aab0a98f050ec232d7b8ce024240020d0