Fix: event notifier: notification send should be a callback

Fix: event notifier: notification send should be a callback

Turn notification send into a callback so probes compiled against
lttng-ust 2.13+ but executed in an environment with lttng-ust 2.12 or
prior don't trigger linker error due to missing
lttng_event_notifier_notification_send symbol.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib51fdc09865f8ae158129417b9e26e86221881b0