extension_board.h: Add missing <linux/list.h>

This file has "struct list" in one of the structures, so add
<linux/list.h>.

Signed-off-by: Tom Rini <trini@konsulko.com>

include/extension_board.h

@@ -7,6 +7,8 @@
 #ifndef __EXTENSION_SUPPORT_H
 #define __EXTENSION_SUPPORT_H
 
+#include <linux/list.h>
+
 struct extension {
 	struct list_head list;
 	char name[32];