trace: Fix sched load balance trace events compilation for non-smp

Commit 'dcbbf13d("Sched: Improve load balancer tracing support")'
introduced new sched load balance trace events, which are applicable
for only when SMP enabled.

Move them under CONFIG_SMP to fix compilation issue for non-smp targets.

Change-Id: I008c956aabbe241640971a485703a919264faacb
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>