lisa.tests.scheduler.eas_behaviour: Fix _get_rtapp_profile() prototype

commit 487e1ac4 ("lisa.tests.scheduler.eas_behaviour:
Add big-task-duty-cycle platform parameter")
modified the prototype of _get_rtapp_profile(), but an implementation
was left with the initial prototype, triggering errors. Fix that.

Reported-by: Deepak Kumar Mishra <deepakkumar.mishra@arm.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>