watchdog: i6300esb: fix build failure

i6300esb uses fuctions defined in watchdog_core.c, and when
CONFIG_WATCHDOG_CORE is not set we have this build error:

drivers/watchdog/i6300esb.o: In function `esb_remove':
i6300esb.c:(.text+0xcc): undefined reference to `watchdog_unregister_device'
drivers/watchdog/i6300esb.o: In function `esb_probe':
i6300esb.c:(.text+0x2a1): undefined reference to `watchdog_init_timeout'
i6300esb.c:(.text+0x388): undefined reference to `watchdog_register_device'
make: *** [Makefile:1029: vmlinux] Error 1

Fix this by selecting CONFIG_WATCHDOG_CORE when I6300ESB_WDT is set.

Fixes: 7af4ac87 ("watchdog: i6300esb: use the watchdog subsystem")
Signed-off-by: Matteo Croce <mcroce@redhat.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>