Fix: correct mismatched function signatures

The extern declaration of `_lttng_create_session_ext` in `create.c`
had a superfluous `live_timer` parameter not present in the actual
function definition in `lttng_ctl.c`. The -1 value with which it was
called was therefore unused.

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