util: Look for newer name of cpu wait time statistic

It looks like linux changed the key for wait time in /proc/<pid>/sched
and /proc/<pid>/task/<tid>/sched files in commit ceeadb83aea2 (or around
that time) from se.statistics.wait_sum to just wait_sum.  Similarly to
the previous change (from se.wait_sum) just look for the new name first.

Resolves: https://issues.redhat.com/browse/RHEL-60030


Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>