drivers: soc: qcom: Fix per_cpu declaration of pending_ipi

Switch an accidental use of DEFINE_PER_CPU when
the developer clearly wanted to use DECLARE_PER_CPU.

CONFIG_DEBUG_FORCE_WEAK_CPU enforces that each per-cpu object
in the kernel have a unique name so if one accidently uses
DEFINE_PER_CPU_READ_MOSTLY in a header it would define the same
symbol in each file that used the header.

Fixes: 0aaf4cf7 ("drivers: soc: qcom: snapshot of idle/sleep driver as of msm-4.14")
Change-Id: Ic0dedbadbce4e85f2130ea59c84d1e10767d50d7
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>