xen/tmem: mark xen_tmem_init() __init

As a module_init() function, this should have been this way from the
beginning.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/tmem.c

@@ -374,7 +374,7 @@ static struct frontswap_ops tmem_frontswap_ops = {
 };
 #endif
 
-static int xen_tmem_init(void)
+static int __init xen_tmem_init(void)
 {
 	if (!xen_domain())
 		return 0;