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: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>