ios-rust: Ignore generated UniFFI sources.

As of mozilla/application-services#6413, `MozillaTestServices`
outputs all generated sources into `MozillaTestServices/Generated`.
Let's ignore them, so that they don't show up as untracked in Git.

.gitignore

@@ -38,7 +38,7 @@ megazords/ios-rust/focus/FocusRustComponents.xcframework*
 
 
 # Glean generated artifacts
-megazords/ios-rust/MozillaTestServices/MozillaTestServices/Generated/Metrics.swift
+megazords/ios-rust/MozillaTestServices/MozillaTestServices/Generated
 megazords/ios-rust/MozillaTestServices/.venv
 
 # Carthage generated artifacts