Fix: lttng-ust 2.13 should not try to use notifiers from 2.12 or prior probes

Fix: lttng-ust 2.13 should not try to use notifiers from 2.12 or prior probes

Probe providers built against lttng-ust 2.12 or prior do not implement
the event notifier callback, and accessing desc->u.ext.event_notifier_callback
accesses beyond the size of struct lttng_event_desc.

Therefore, skip those older probe providers from the list of providers
considered for event notifications, and print an error when this
situation is encountered.

Currently, the error reporting of lttng-ust is performed through ERR(),
which is only observable on the console of the application when run with
LTTNG_UST_DEBUG=1.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ia8a9bae1a471ae5d30ff0364b28e1cd3a8b4e396