rtc: ds1374: fix RTC_DRV_DS1374_WDT dependencies

It is not enough to select WATCHDOG_CORE, the watchdog part of the driver
now also depends on WATCHDOG. This is currently the best we can do because
alarm support and watchdog support are mutually exclusive.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20200720075250.1019172-1-alexandre.belloni@bootlin.com