GKI: change config option back to PREEMPT

b8d33498 ("sched/rt, Kconfig: Unbreak def/oldconfig with
CONFIG_PREEMPT=y") fixed up the problem with the change in the PREEMPT
config option, so put it back to what it was as "voluntary" was not
correct.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I61d3a772f27edae401e785753c5b6a7559b8839b