util: add common ARRAY_SIZE define

I want to use this in more places, so put it in util.h rather than
copying & pasting it into another file.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.h

@@ -24,6 +24,8 @@
  *                                                                   USA
  */
 
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+
 static inline void __attribute__((noreturn)) die(const char *str, ...)
 {
 	va_list ap;