kconfig: fix lists definition for C++

The C++ compiler is more strict in that it refuses to assign
a void* to a struct list_head*.

Fix that by explicitly casting the poisonning constants.

(Tested with all 5 frontends, now.)
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: N"Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Benjamin Poirier <bpoirier@suse.de>

scripts/kconfig/list.h

@@ -125,7 +125,7 @@ static inline void __list_del(struct list_head *prev, struct list_head *next)
 static inline void list_del(struct list_head *entry)
 {
 	__list_del(entry->prev, entry->next);
-	entry->next = LIST_POISON1;
-	entry->prev = LIST_POISON2;
+	entry->next = (struct list_head*)LIST_POISON1;
+	entry->prev = (struct list_head*)LIST_POISON2;
 }
 #endif