DM: use standard offsetof to avoid duplicate definition

Signed-off-by: Liu Shuo <shuo.a.liu@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>

Author: Liu Shuo

devicemodel/hw/platform/rtc.c

@@ -30,6 +30,7 @@
 #include <pthread.h>
 #include <string.h>
 #include <assert.h>
+#include <stddef.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -49,8 +50,6 @@
 # define RTC_DEBUG(format, ...)      do { } while (0)
 #endif
 
-#define offsetof(type, member)	((size_t)(&((type *)0)->member))
-
 /* Register layout of the RTC */
 struct rtcdev {
 	uint8_t	sec;