[IA64] Include Kconfig.preempt

arch/ia64/Kconfig failed to include kernel/Kconfig.preempt that meant it
did not support PREEMPT_VOLUNTARY and PREEMPT_BKL (inadvertently).
This was recently noticed when the newly-added PREEMPT_NOTIFIERS in
Kconfig.preempt that was "select"ed from drivers/kvm/Kconfig (therefore)
started giving bogus warnings ('select' used by config symbol 'KVM' refers
to undefined symbol 'PREEMPT_NOTIFIERS') on ia64 builds.

So let's remove the open-coded definition of CONFIG_PREEMPT in
arch/ia64/Kconfig and replace it with just including Kconfig.preempt
instead, like the other archs do.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>