sched: Fix per-cpu declaration of sched_load_boost

Switch an accidental use of DEFINE_PER_CPU_READ_MOSTLY when
the developer clearly wanted to use DECLARE_PER_CPU_READ_MOSTLY.

CONFIG_DEBUG_FORCE_WEAK_CPU enforces that each per-cpu object
in the kernel have a unique name so if ccidently uses
DEFINE_PER_CPU_READ_MOSTLY in a header it would define the same
symbol in each file that used the header which in the case of
linux/sched.h is pretty much all of them.

Fixes: 7ebdf76d ("sched: Add snapshot of Window Assisted Load Tracking (WALT)")
Change-Id: Ic0dedbad780bfce92aee3b79aec0fa99155f66be
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>