sched/fair: Add rcu_read_lock() call in update_runtime_enabled()

Commit 7ebdf76d ("sched: Add snapshot of Window Assisted Load
Tracking (WALT)") removed rcu_read_lock() call from
update_runtime_enabled() which would lead to issues, fix it by
adding rcu_read_lock() call.

Change-Id: I3853fa31009da596e23fd559c55bc5c261fd0e0d
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>