Require only app-extension-safe API issue.

[xieweizhi]

When working with app extensions, need to set the pod Require Only App-Extension-Safe API to No.

But this will raise a warning ld: warning: linking against a dylib which is not safe for use in application extensions

I found a solution https://miqu.me/blog/2016/11/28/app-extensions-xcode-and-cocoapods-omg/