Compiler warning at /kernel/sched.c

[spoorthik]

Log:

/home/kspoorth/work/latest_zephyr/kernel/sched.c: In function '_reschedule':
/home/kspoorth/work/latest_zephyr/kernel/sched.c:448:9: warning: 'return' with a value, in function returning void
  return _Swap(key);
         ^

It is good to get rid of this as it would be an issue while building with compiler warnings being treated as errors.