Don't call acct_update_power on ARCH=um.

kernel/sched/core.c calls acct_update_power, but that function is
only compiled if CONFIG_CPU_FREQ_STAT is enabled. This breaks the
ability to run kernel unit tests because the kernel won't link
on ARCH=um. #ifdef out the code on ARCH=um.

BUG=21498425
BUG=23016816
Change-Id: I7a95c842ac7eb9017d5e2ff554c9ab8a011a3e4d
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>