xen/pv-on-hvm:kexec: Fix implicit declaration of function 'xen_hvm_domain'

Randy found a compile error when using make randconfig to trigger

drivers/xen/xenbus/xenbus_xs.c:909:2: error: implicit declaration of function 'xen_hvm_domain'

it is unclear which of the CONFIG options triggered this. This
patch fixes the error.
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_xs.c

@@ -45,6 +45,7 @@
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <xen/xenbus.h>
+#include <xen/xen.h>
 #include "xenbus_comms.h"
 
 struct xs_stored_msg {