progs/perf: offsetof() should be defined in stddef.h, include it

If this breaks mingw, feel free to revert this.
diff --git a/progs/perf/teximage.c b/progs/perf/teximage.c
index 11d781f..634cd83 100644
--- a/progs/perf/teximage.c
+++ b/progs/perf/teximage.c
@@ -50,11 +50,8 @@
    { 0.0, 0.0, 0.5, 0.5 },
 };
 
-#if 0
 #define VOFFSET(F) ((void *) offsetof(struct vertex, F))
-#else
-#define VOFFSET(F) ((void *) &((struct vertex *)NULL)->F)
-#endif
+
 
 /** Called from test harness/main */
 void