sched: debug: fix sum_exec_runtime clearing

when cleaning sched-stats also clear prev_sum_exec_runtime.

Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched_debug.c

···
 	p->se.wait_runtime_overruns = p->se.wait_runtime_underruns = 0;
 #endif
 	p->se.sum_exec_runtime = 0;
+	p->se.prev_sum_exec_runtime	= 0;
 }