FIXUP: sched/tune: properly account of exiting tasks (only for kernel < v4.4)

Once a task is going to end the CGroup::exit() callback is required to
properly update the counting of tasks active in the original boostgroup.

NOTE: This callback is NOT required after the API changes introduced
      in kernels >=v4.4 by this patch:

2e91fa7f cgroup: keep zombies associated with their original cgroups

Change-Id: I4dc6b77a9621003ac2269f30ea44c8b024fbc249
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>