Fix potential Equatable issue on future releases

[pacu]

When running sh Scripts/build_swift.sh

Building for debugging...
/frost-uniffi-sdk/FrostSwift/Sources/FrostSwift/Extensions/FrostSwift+Orchard.swift:10:1: warning: extension declares a conformance of imported type 'OrchardFullViewingKey' to imported protocol 'Equatable'; this will not behave correctly if the owners of 'FrostSwiftFFI' introduce this conformance in the future
 8 | import FrostSwiftFFI
 9 | 
10 | extension OrchardFullViewingKey: Equatable {
   | |- warning: extension declares a conformance of imported type 'OrchardFullViewingKey' to imported protocol 'Equatable'; this will not behave correctly if the owners of 'FrostSwiftFFI' introduce this conformance in the future
   | `- note: add '@retroactive' to silence this warning