mei: fix compilation error with missing WATCHDOG_CORE

Kconfig is not transitive so INTEL_ME_TXE has to depend
on WATCHDOG_CORE as well

ERROR: "watchdog_unregister_device" [drivers/misc/mei/mei.ko] undefined!
ERROR: "watchdog_register_device" [drivers/misc/mei/mei.ko] undefined!

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>