[WATCHDOG] Fix build with CONFIG_ITCO_VENDOR_SUPPORT=n

The problem is that iTCO_vendor_support.ko is still being built while
iTCO_vendor.h claims that its functions do not exist. The following
Makefile update fixes that. It causes iTCO_vendor_support.ko to no
longer be built if CONFIG_ITCO_VENDOR_SUPPORT=n.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>