fix for swift 5 linker
fix for swift 5 linker

This is necessary for `swift test` to link and run.
It's unclear why this particular designated initializer needs to be overridden for the linker to be happy; there are two more, public, designated initializers in `Deferred` that cause no problem for the linker.

glessard/swiftian-dispatch,glessard/deferred,glessard/deferred,glessard/async-deferred