Fix: uninitialized variable in lib_ring_buffer_channel_switch_timer_start

Fix: uninitialized variable in lib_ring_buffer_channel_switch_timer_start

Found by Coverity:
** CID 1447027:  Uninitialized variables  (UNINIT)
/libringbuffer/ring_buffer_frontend.c: 810 in lib_ring_buffer_channel_switch_timer_start()

>>>     CID 1447027:  Uninitialized variables  (UNINIT)
>>>     Using uninitialized value "sev". Field "sev._sigev_un" is uninitialized when calling "timer_create".

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Iaa66e5612ff1c51b50c4c0b8f30f3695e1b03153