xen/xenbus-frontend: Fix compile error with randconfig

drivers/xen/xenbus/xenbus_dev_frontend.c: In function 'xenbus_init':
drivers/xen/xenbus/xenbus_dev_frontend.c:609:2: error: implicit declaration of function 'xen_domain'

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -58,6 +58,7 @@
 #include "xenbus_comms.h"
 
 #include <xen/xenbus.h>
+#include <xen/xen.h>
 #include <asm/xen/hypervisor.h>
 
 MODULE_LICENSE("GPL");