sched: Fix compile error

sched_get_busy(), sched_set_io_is_busy() and sched_set_window() need
to be defined only when CONFIG_SCHED_FREQ_INPUT is defined, otherwise
we get compilation error related to dual definition of those routines

Change-Id: Ifd5c9b6675b78d04c2f7ef0e24efeae70f7ce19b
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>