x86: lguest build fix

Fix this error (i386 !SMP build):

arch/x86/lguest/boot.c: In function lguest_init:
arch/x86/lguest/boot.c:1059: error: pm_power_off undeclared (first use in this function)

by including linux/pm.h.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>