sched_ext: idle: Make local functions static in ext_idle.c

commit 353656eb upstream.

Functions that are only used within ext_idle.c can be marked as static
to limit their scope.

No functional changes.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>