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>