函数 objc_getAssociatedObject 的隐式声明在 C99 无效

我正在尝试在我的 Objective-c 类别中使用关联对象，但收到此警告.

I'm trying to use Associated Objects on my objective-c category and I'm getting this warning.

Implicit declaration of function 'objc_getAssociatedObject' is invalid in C99

如果这有所不同，我也会使用 arc.

Also I'm using arc if that makes a difference.