rtc: export rtc_month_days()

Export function rtc_month_days() for reuse in the UEFI subsystem.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
index 6528ddf..1f7bdad 100644
--- a/drivers/rtc/rtc-lib.c
+++ b/drivers/rtc/rtc-lib.c
@@ -23,7 +23,7 @@
 /*
  * The number of days in the month.
  */
-static int rtc_month_days(unsigned int month, unsigned int year)
+int rtc_month_days(unsigned int month, unsigned int year)
 {
 	return rtc_days_in_month[month] + (is_leap_year(year) && month == 1);
 }