xen/tmem: Fix compile warning.

We keep on getting:
drivers/xen/tmem.c:65:13: warning: ‘disable_frontswap_selfshrinking’ defined but not used [-Wunused-variable]

if CONFIG_FRONTSWAP=y and # CONFIG_CLEANCACHE is not set

Found by 0 day test project
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/tmem.c

@@ -61,14 +61,12 @@ __setup("nofrontswap", no_frontswap);
 #endif
 #endif /* CONFIG_FRONTSWAP */
 
-#ifdef CONFIG_FRONTSWAP
+#ifdef CONFIG_XEN_SELFBALLOONING
 static bool disable_frontswap_selfshrinking __read_mostly;
 #ifdef CONFIG_XEN_TMEM_MODULE
 module_param(disable_frontswap_selfshrinking, bool, S_IRUGO);
-#else
-#define disable_frontswap_selfshrinking 1
 #endif
-#endif /* CONFIG_FRONTSWAP */
+#endif /* CONFIG_XEN_SELFBALLOONING */
 
 #define TMEM_CONTROL               0
 #define TMEM_NEW_POOL              1