Actually that line already exists, but it's ifdef'd out incorrectly to play nice with Windows. Change it to:
#if UNITY_EDITOR_OSX || (UNITY_EDITOR && UNITY_IOS)
using UnityEditor.iOS.Xcode;
#endif
Note also that fixing this is a temporary/unstable solution: If you ever wipe your Library/PackageCache folders (which happens frequently in dev practice), then this .cs file will get generated with the error again.
To properly fix this, it should be addressed in the github project (I left feedback via the apple site as the github project directs, I suggest if you're seeing this problem you do the same)
glhfbiz
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You have to build these plugins as packages on a MacOS machine, afaik.
The problem occurs when you, for example, submit to a source control repo, and then pull to a Windows machine and try to open in Unity, even if you're in iOS build mode.