Silence unchecked warning

Was:

    [warn] /Users/jason/code/dotty/src/dotty/tools/dotc/printing/PlainPrinter.scala:107: non-variable type argument dotty.tools.dotc.core.Types.RefinedType in type pattern List[dotty.tools.dotc.core.Types.RefinedType] is unchecked since it is eliminated by erasure
    [warn]         val parent :: (refined: List[RefinedType]) =
    [warn]                                 ^