sched: define sched_set_boost API even when WALT is enabled

As cpu-boost has reference to sched_set_boost and is enabled,
need sched_set_boost definition to enable WALT.

Change-Id: Ic1acc4a43c72a9e27bd6e9840d82f39357cecd84
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>