watchdog: fix compile time error of pretimeout governors

CONFIG_WATCHDOG_PRETIMEOUT_GOV build symbol adds watchdog_pretimeout.o
object to watchdog.o, the latter is compiled only if CONFIG_WATCHDOG_CORE
is selected, so it rightfully makes sense to add it as a dependency.

The change fixes the next compilation errors, if CONFIG_WATCHDOG_CORE=n
and CONFIG_WATCHDOG_PRETIMEOUT_GOV=y are selected:

  drivers/watchdog/pretimeout_noop.o: In function `watchdog_gov_noop_register':
  drivers/watchdog/pretimeout_noop.c:35: undefined reference to `watchdog_register_governor'
  drivers/watchdog/pretimeout_noop.o: In function `watchdog_gov_noop_unregister':
  drivers/watchdog/pretimeout_noop.c:40: undefined reference to `watchdog_unregister_governor'

  drivers/watchdog/pretimeout_panic.o: In function `watchdog_gov_panic_register':
  drivers/watchdog/pretimeout_panic.c:35: undefined reference to `watchdog_register_governor'
  drivers/watchdog/pretimeout_panic.o: In function `watchdog_gov_panic_unregister':
  drivers/watchdog/pretimeout_panic.c:40: undefined reference to `watchdog_unregister_governor'

Reported-by: Kuo, Hsuan-Chi <hckuo2@illinois.edu>
Fixes: ff84136c ("watchdog: add watchdog pretimeout governor framework")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.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@linux-watchdog.org>