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: 6256f7f7 ("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.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-omap.c

@@ -25,6 +25,7 @@
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
 #include <linux/rtc.h>
+#include <linux/rtc/rtc-omap.h>
 
 /*
  * The OMAP RTC is a year/month/day/hours/minutes/seconds BCD clock