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: 121f2fa ("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>

drivers/xen/balloon.c

@@ -80,9 +80,8 @@
 static uint __read_mostly balloon_boot_timeout = 180;
 module_param(balloon_boot_timeout, uint, 0444);
 
-static int xen_hotplug_unpopulated;
-
 #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
+static int xen_hotplug_unpopulated;
 
 static struct ctl_table balloon_table[] = {
 	{
@@ -115,6 +114,8 @@ static struct ctl_table xen_root[] = {
 	{ }
 };
 
+#else
+#define xen_hotplug_unpopulated 0
 #endif
 
 /*