xen/self-balloon: Add dependency on tmem.

Without enabling CONFIG_XEN_TMEM we get this:

drivers/xen/xen-selfballoon.c:461: undefined reference to `tmem_enabled'
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/Kconfig

@@ -11,7 +11,7 @@ config XEN_BALLOON
 
 config XEN_SELFBALLOONING
 	bool "Dynamically self-balloon kernel memory to target"
-	depends on XEN && XEN_BALLOON && CLEANCACHE && SWAP
+	depends on XEN && XEN_BALLOON && CLEANCACHE && SWAP && XEN_TMEM
 	default n
 	help
 	  Self-ballooning dynamically balloons available kernel memory driven