check_software: tolerate libthread_db from system

this is part of libc
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:00:33

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:30

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:27