xen/balloon: fix unused-variable warning

xen/balloon: fix unused-variable warning

In configurations with CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=n
and CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y, gcc warns about an
unused variable:

drivers/xen/balloon.c:83:12: error: 'xen_hotplug_unpopulated' defined but not used [-Werror=unused-variable]

Since this is always zero when CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
is disabled, turn it into a preprocessor constant in that case.

Fixes: 121f2faca2c0 ("xen/balloon: rename alloc/free_xenballooned_pages")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20211108111408.3940366-1-arnd@kernel.org
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>