As per http://lkml.org/lkml/2003/12/1/150

diff -urN --exclude=CVS --exclude=.cvsignore linux-2.4.23/include/linux/mc146818rtc.h linux-cvs-2.4.23/include/linux/mc146818rtc.h
--- linux-2.4.23/include/linux/mc146818rtc.h	2001-11-22 20:46:58.000000000 +0100
+++ linux-cvs-2.4.23/include/linux/mc146818rtc.h	2003-11-28 15:09:41.000000000 +0100
@@ -98,4 +98,12 @@
 #define BIN_TO_BCD(val) ((val)=(((val)/10)<<4) + (val)%10)
 #endif
 
+#ifndef RTC_IO_EXTENT
+#define RTC_IO_EXTENT	0x10	/* Only really two ports, but...	*/
+#endif
+
+#ifndef RTC_IOMAPPED
+#define RTC_IOMAPPED	1	/* Default to I/O mapping. */
+#endif
+
 #endif /* _MC146818RTC_H */