xen/balloon: add header guard

In order to protect against the header being included multiple times
on the same compilation unit.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20200901083326.21264-2-roger.pau@citrix.com


Signed-off-by: Juergen Gross <jgross@suse.com>