sched/walt: fix compilation issues for !CONFIG_ENERGY_MODEL

When CONFIG_ENERGY_MODEL is not selected, compilation issues
are observed. Fix by adding check for CONFIG_ENERGY_MODEL.

Change-Id: Iafbf40596677f75e2425debb96e23d42301a9c0e
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>