lisa.datautils: Add SignalDesc for sched_pelt_thermal event

This is necessary to have a correct windowing behavior, i.e. to have an event
at the beginning of any window if the information exists. Otherwise analysis
like taking an average value or plotting a signal will be somehow broken when
using trace windows.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>