xen/balloon: Fix declared-but-not-defined warning

Fix a declared-but-not-defined warning when building with
XEN_BALLOON_MEMORY_HOTPLUG=n. This fixes a regression introduced by
commit dfd74a1e

 ("xen/balloon: Fix crash when ballooning on x86 32
bit PAE").

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>