lttng-ctl: Introduce lttng_log_level_rule

This object corresponds to the criteria a event-rule tracepoint must
respect for an event to be consired under the event-rule tracepoint
criteria.

The lttng tracepoint event rule object now have the following API
function in regards to log level rule:

  lttng_event_rule_tracepoint_set_log_level_rule
  lttng_event_rule_tracepoint_get_log_level_rule

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iefd361d1b7d85c6803e891166429a2830e2404a2