watchdog: xen_wdt: fix potential build failure

xen_wdt uses watchdog core functions (from watchdog_core.c) and, when
compiled without CONFIG_WATCHDOG_CORE being set, it produces the
following build error:

ERROR: "devm_watchdog_register_device" [drivers/watchdog/xen_wdt.ko] undefined!
ERROR: "watchdog_init_timeout" [drivers/watchdog/xen_wdt.ko] undefined!

Fix this by selecting CONFIG_WATCHDOG_CORE when CONFIG_XEN_WDT is set.

Fixes: 18cffd68 ("watchdog: xen_wdt: use the watchdog subsystem")
Signed-off-by: Radu Rendec <radu.rendec@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>