rtc: omap: include header for omap_rtc_power_off_program prototype

rtc: omap: include header for omap_rtc_power_off_program prototype

Non-static functions should have a prototype:

  drivers/rtc/rtc-omap.c:410:5: error: no previous prototype for ‘omap_rtc_power_off_program’ [-Werror=missing-prototypes]

Fixes: 6256f7f7f217 ("rtc: OMAP: Add support for rtc-only mode")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230311094021.79730-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>