staging: lttng: TODO update: lttng reported to work fine on -rt now

The patch "lttng: Fix recent modifications to string_from_user
operation" has been confirmed to fix the corrupted trace problem
experienced on -rt kernel by the original bug reporter. Remove the
entry from the LTTng TODO list.

Reported-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>