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.

---
Gee, lguest appears to be unmaintained, according to MAINTAINERS :)

index d2235db..8f13562 100644
@@ -56,6 +56,7 @@
 #include <linux/lguest.h>
 #include <linux/lguest_launcher.h>
 #include <linux/virtio_console.h>
+#include <linux/pm.h>
 #include <asm/paravirt.h>
 #include <asm/param.h>
 #include <asm/page.h>