xmem/tmem: fix 'undefined variable' build error.

In the (not so useful) kernel configuration where CONFIG_SWAP
is undefined and CONFIG_XEN_SELFBALLOONING is defined,
xen_tmem_init would use undefined variable 'static bool frontswap'.

Added #else to have #define frontswap (0) in the case where
CONFIG_FRONTSWAP is not defined.

Signed-off-by: Frederico Cadete <frederico@cadete.eu>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/tmem.c

@@ -41,6 +41,8 @@ module_param(selfballooning, bool, S_IRUGO);
 #ifdef CONFIG_FRONTSWAP
 static bool frontswap __read_mostly = true;
 module_param(frontswap, bool, S_IRUGO);
+#else /* CONFIG_FRONTSWAP */
+#define frontswap (0)
 #endif /* CONFIG_FRONTSWAP */
 
 #ifdef CONFIG_XEN_SELFBALLOONING