Fix: ensure a newline is printed before exit on client destroy

Errors encountered during the destruction of a session can cause
the `lttng` client to exit after having printed the
'Waiting for destruction of session ...' message without printing
a newline. This causes the user's prompt to reappear after the
message and not on a new line.

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