tools/libs/vchan: remove private offsetof() definition

vchan/init.c is defining offsetof privately. Remove that definition
and just use stddef.h instead.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

tools/libs/vchan/init.c

@@ -32,6 +32,7 @@
 #include <sys/mman.h>
 #include <sys/ioctl.h>
 #include <sys/user.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdint.h>
@@ -69,10 +70,6 @@
 #define MAX_RING_SHIFT 20
 #define MAX_RING_SIZE (1 << MAX_RING_SHIFT)
 
-#ifndef offsetof
-#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
-#endif
-
 static int init_gnt_srv(struct libxenvchan *ctrl, int domain)
 {
 	int pages_left = ctrl->read.order >= PAGE_SHIFT ? 1 << (ctrl->read.order - PAGE_SHIFT) : 0;