FIXUP: sched/tune: fix compilation error on !CONFIG_CGROUP_SCHEDTUNE

When SchedTune is configured without CGroups support, the schedtune_accept_deltas()
function is not declared.
Since schedule candidate filtering is required also when global boosting
mode is in use, this patch move the declaration of that function to make
it dependant just on CONFIG_SCHED_TUNE.

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