Fix postfix warning in project/Fmpp.scala

[xuwei-k]

[warn] This can be achieved by adding the import clause 'import scala.language.postfixOps'
[warn] or by setting the compiler option -language:postfixOps.
[warn] See the Scaladoc for value scala.language.postfixOps for a discussion
[warn] why the feature should be explicitly enabled.
[warn]   lazy val FmppConfig = config("fmpp") hide
[warn]                                        ^