xen: self-balloon needs module.h

Fix build errors (found when CONFIG_SYSFS is not enabled):

  drivers/xen/xen-selfballoon.c:446: warning: data definition has no type or storage class
  drivers/xen/xen-selfballoon.c:446: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL'
  drivers/xen/xen-selfballoon.c:446: warning: parameter names (without types) in function declaration
  drivers/xen/xen-selfballoon.c:485: error: expected declaration specifiers or '...' before string constant
  drivers/xen/xen-selfballoon.c:485: warning: data definition has no type or storage class
  drivers/xen/xen-selfballoon.c:485: warning: type defaults to 'int' in declaration of 'MODULE_LICENSE'
  drivers/xen/xen-selfballoon.c:485: warning: function declaration isn't a prototype

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>