Apple's CryptoTokenKit is only present in iOS 13.0+ however it exports
symbols with availability annotations going back to iOS 10.0.
In the Pass app we have a deployment target of iOS 12.0. Apple's
automatic weak linking system apparently only looks at the
symbol-level availability annotations so it assumes the symbols
we use will always be present (even though they won't pre-iOS-13).
We can work around this issue by forcing weak linking using the
"Optional" framework setting. (Note that this workaround would not
work if CryptoTokenKit was used from a third-party swift package.)
This is necessary to restore iOS 12 support after #533.
For further history see https://github.com/mssun/passforios/issues/539
