libcutils: Clarify schedboost_enabled()

Commit 795267d4 ("Removed cpusets/schedboost build time dependency.")
turned the cpusets and schedtune options into runtime
decisions.

However the kernel option which is mentioned in the
code comment (CONFIG_SCHEDTUNE) is very misleading
as it doesn't exist (CONFIG_SCHED_TUNE does exist)
and it doesn't describe the real functionality of
the method. schedboost_enabled() will still return
false if CONFIG_SCHED_TUNE is set in the kernel but
CONFIG_CGROUP_SCHEDTUNE is not.

So to clarify this, we need to change the comment
to reflect that CONFIG_CGROUP_SCHEDTUNE, which depends
on CONFIG_SCHED_TUNE, is required.

Signed-off-by: Alex Naidis <alex.naidis@linux.com>