Bug fix

This fix removes this exception
build.sbt:16: error: No implicit for Append.Value[Seq[sbt.Task[Seq[java.io.File]]], Seq[sbt.File]] found,
  so Seq[sbt.File] cannot be appended to Seq[sbt.Task[Seq[java.io.File]]]
    sourceGenerators in Compile += (scalaxb in Compile).value,