xen: fix build breakage in xen-selfballoon.c caused by sysdev conversion

This adds the needed include file for xen-selfballoon.c to fix the build
error reported by Stephen Rothwell.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/xen/xen-selfballoon.c

@@ -74,6 +74,7 @@
 #include <linux/mman.h>
 #include <linux/module.h>
 #include <linux/workqueue.h>
+#include <linux/device.h>
 #include <xen/balloon.h>
 #include <xen/tmem.h>
 #include <xen/xen.h>