Fix: action: lttng_action_group_add_action returns status invalid

Fix: action: lttng_action_group_add_action returns status invalid

IS_GROUP_ACTION macro was performed on the action to be added instead
of the action representing the group, `group` in this function.

This always resulted in error on call to lttng_action_group_add_action.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iecf133fb2165686d1e8abd2225a8f90df22c41dc