simpleperf: fix excluding kernel callchains when --trace-offcpu is used.

When recording with "-e cpu-cycles:u --trace-offcpu", the sched_switch
records are wrongly skipped.

Bug: None
Test: run python test.py TestExamplePureJavaTraceOffCpu
Test: run simpleperf_unit_test
Change-Id: I086e2f39b40be75146d52db68977053d7d75d8e9
1 file changed