update MiMa whitelist for infix annotation

[SethTisue]

little missing piece of #5589

[SethTisue]

fixes the failure seen at e.g. https://scala-ci.typesafe.com/job/scala-2.12.x-integrate-bootstrap/806/consoleFull

it also made one of my own PRs fail validation, which leaves me even more puzzled why this wasn't caught during 5589's own validation oh wait, this was already covered at #5589 (comment). so this is expected, it's because 5589 wasn't rebased before merge

[SethTisue]

Internal error when running tests: sbt.ForkMain$Run$RunAborted: java.net.SocketException: Broken pipe (Write failed)
[error] Error: Total 0, Failed 0, Errors 0, Passed 0
[error] Error during tests:
[error] 	Forked test harness failed: java.lang.NullPointerException
[error] 	at sbt.TestEvent$$anonfun$overallResult$1.apply(TestReportListener.scala:72)
[error] 	at sbt.TestEvent$$anonfun$overallResult$1.apply(TestReportListener.scala:71)
[error] 	at scala.collection.IndexedSeqOptimized$class.foldl(IndexedSeqOptimized.scala:51)
[error] 	at scala.collection.IndexedSeqOptimized$class.foldLeft(IndexedSeqOptimized.scala:60)
[error] 	at scala.collection.mutable.WrappedArray.foldLeft(WrappedArray.scala:34)
[error] 	at scala.collection.TraversableOnce$class./:(TraversableOnce.scala:138)
[error] 	at scala.collection.AbstractTraversable./:(Traversable.scala:105)
[error] 	at sbt.TestEvent$.overallResult(TestReportListener.scala:71)
[error] 	at sbt.TestEvent$$anon$1.<init>(TestReportListener.scala:66)
[error] 	at sbt.TestEvent$.apply(TestReportListener.scala:65)
[error] 	at sbt.React.react(ForkTests.scala:137)
[error] 	at sbt.ForkTests$$anonfun$mainTestTask$1$Acceptor$2$.run(ForkTests.scala:76)
[error] 	at java.lang.Thread.run(Thread.java:745)

[SethTisue]

/rebuild

[lrytz]

LGTM