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: Rusty Russell <rusty@rustcorp.com.au>