Add support for the DS3231 RTC


diff --git a/rtc/Makefile b/rtc/Makefile
index cf2b24e..cdc8ac9 100644
--- a/rtc/Makefile
+++ b/rtc/Makefile
@@ -29,7 +29,7 @@
 
 COBJS	= date.o   \
 	  bf533_rtc.o ds12887.o ds1302.o ds1306.o ds1307.o \
-	  ds1337.o ds1374.o ds1556.o ds164x.o ds174x.o \
+	  ds1337.o ds1374.o ds1556.o ds164x.o ds174x.o ds3231.o \
 	  m41t11.o max6900.o m48t35ax.o mc146818.o mk48t59.o \
 	  mpc5xxx.o mpc8xx.o pcf8563.o s3c24x0_rtc.o rs5c372.o