xen/xenbus: Fix compile error - missing header for xen_initial_domain()

drivers/xen/xenbus/xenbus_dev_backend.c:74:2: error: implicit declaration of function 'xen_initial_domain'
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_dev_backend.c

@@ -6,6 +6,7 @@
 #include <linux/module.h>
 #include <linux/capability.h>
 
+#include <xen/xen.h>
 #include <xen/page.h>
 #include <xen/xenbus_dev.h>