Fix: lttng: add-trigger: leak of error string

argpar_state_parse_next allocates an error string on
`ARGPAR_STATE_PARSE_NEXT_STATUS_ERROR_UNKNOWN_OPT` which we choose not
to handle as an error. Still, the error must be free'd.

1447048 Resource leak

The system resource will not be reclaimed and reused, reducing the future availability of the resource.

In handle_action_simple_session: Leak of memory or pointers to system resources (CWE-404)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib4495dedc6273f376f93f9c5e2a26baf1c08e16c