sched: compile sched_avg.c only for SMP

The stats exported by sched_avg.c like running average of
runnable tasks and CPU load are used to manage the number
of active CPUs in the system. There is no use case for this
on UP system.

Change-Id: I09c454e742e78813ddcf4ac98858b77970eeaa1f
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>