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>