autodata: fix example, autodata_get()s second arg must be a (size_t *)

CC: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Cody P Schafer <dev@codyps.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/autodata/autodata.h

@@ -59,7 +59,8 @@
  * Example:
  *	static void print_embedded_names(void)
  *	{
- *		unsigned int i, num;
+ *		unsigned int i;
+ *		size_t num;
  *		char **n = autodata_get(names, &num);
  *
  *		for (i = 0; i < num; i++)