Add includes and split off json_types.h to help ensure that headers can be included in any order.

json_object_iterator.h

@@ -23,16 +23,9 @@
 #ifndef JSON_OBJECT_ITERATOR_H
 #define JSON_OBJECT_ITERATOR_H
 
+#include "json_types.h"
 #include <stddef.h>
 
-#ifndef JSON_EXPORT
-#if defined(_MSC_VER)
-#define JSON_EXPORT __declspec(dllexport)
-#else
-#define JSON_EXPORT extern
-#endif
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif