Fix Swift 6 compilation issue

[romanr]

Fixes #3

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[Harlech1]

this doesn't fix :(

[Harlech1]

func generate() async throws {
var result = try await StableDiffusion.predict(with: client, input: .init(prompt: prompt))
try await result.wait(with: client)
prediction = result
}

func cancel() async throws {
guard var result = prediction else { return }
try await result.cancel(with: client)
}

this might work though