Fix: memory leak in lttng_enable_event_with_exclusions

lttng_enable_event_with_exclusions leaks a filter expression when
automatically generated filter statements are used. This happens when
loglevel and logger name filtering are used when enabling JUL events.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>