sched: move up-down migration handler out of CGROUP_SCHED

Below compilation issues are observed when CGROUP_SCHED is
disabled.

kernel/sysctl.o:(.data+0x3e0): \
		undefined reference to `sched_updown_migrate_handler'
kernel/sysctl.o:(.data+0x420): \
		undefined reference to `sched_updown_migrate_handler'

Change-Id: Ia746482b4a48708530788d0ccc187aa9cf03b771
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>