weston_primary_flight_recorder_ring_buffer needs to be cleared on destruction
of the subscriber it was assigned from so that a compositor and be re-executed
in-process (static variables do not get re-initialized automatically).

This will be used by the test harness when it will execute wet_main() multiple
times in the same process. Otherwise it would hit the assert in
weston_log_subscriber_create_flight_rec().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>