alloc: fix missing header in compilation of example.

ccan/alloc/_info

@@ -23,6 +23,7 @@
  *	#include <sys/stat.h>
  *	#include <fcntl.h>
  *	#include <string.h>
+ *	#include <stdlib.h>
  *	#include <ccan/alloc/alloc.h>
  *
  *	static void usage(const char *name)