完善c sdk对象描述元信息，防止隐式指针转换错误

void* 类型指针很容易存在隐式转换错误，用户无法察觉

api/source/mk_player.cpp

@@ -105,7 +105,7 @@ private:
 API_EXPORT mk_player API_CALL mk_player_create() {
     MediaPlayerForC::Ptr *obj = new MediaPlayerForC::Ptr(new MediaPlayerForC());
     (*obj)->setup();
-    return obj;
+    return (mk_player)obj;
 }
 API_EXPORT void API_CALL mk_player_release(mk_player ctx) {
     assert(ctx);