event-notifier: Initialize `_notifier_notification` struct to zero

Valgrind reports the following use of uninitialized bytes:

  ==2233138== Syscall param writev(vector[...]) points to uninitialised byte(s)
  ==2233138==    at 0x4EA161D: __writev (writev.c:26)
  ==2233138==    by 0x4EA161D: writev (writev.c:24)
  ==2233138==    by 0x489A9EC: patient_writev (in /usr/local/lib/liblttng-ust.so.0.0.0)
  ==2233138==    by 0x48C2D5F: lttng_event_notifier_notification_send (in /usr/local/lib/liblttng-ust.so.0.0.0)
  ==2233138==    by 0x4033F3: __event_notifier_probe__tp___tptest (in /home/frdeso/projets/lttng/tools/tests/utils/testapp/gen-ust-events/gen-ust-events)
  ==2233138==    by 0x401D6E: main (in /home/frdeso/projets/lttng/tools/tests/utils/testapp/gen-ust-events/gen-ust-events)
  ==2233138==  Address 0x1ffeffedda is on thread 1's stack
  ==2233138==  in frame #2, created by lttng_event_notifier_notification_send (???:)
  ==2233138==

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I49ac4b440df61b322ac9614b09c32d1f69beedd0