Fix win64 portability warning

Reported by Ozkan Sezer <sezeroz@gmail.com>:

There are also warnings wherever list_entry() macro is used, such as:

src/load.c: In function 'unlink_tempfiles':
src/load.c:404:8: warning: cast from pointer to integer of different size

... or:

options.c: In function 'usage':
options.c:52:8: warning: cast from pointer to integer of different size
options.c:57:8: warning: cast from pointer to integer of different size

... which may be cured by changing (unsigned long) cast with (size_t)

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>

src/list.h

@@ -133,7 +133,7 @@ static __inline__ void list_splice(struct list_head *list, struct list_head *hea
  * @member:	the name of the list_struct within the struct.
  */
 #define list_entry(ptr, type, member) \
-	((type *)((char *)(ptr)-(unsigned long)(&((type *)0)->member)))
+	((type *)((char *)(ptr)-(size_t)(&((type *)0)->member)))
 
 /**
  * list_for_each	-	iterate over a list