drivers: rtc: Fix failing RV8803 driver build when int-gpios not used

This PR fixes the issue that building the driver with either
CONFIG_RTC_CALIBRATION=y or CONFIG_RTC_ALARM=y and without
'int-gpios' used will cause
'undefined reference to `rv8803_write_reg8'' error.

Signed-off-by: Marcin Lyda <elektromarcin@gmail.com>