Make immutable.Set#parCombiner object-protected

This matches every other override of parCombiner in the library and is
necessary to avoid the following variance issue detected by dotty:

src/library/scala/collection/immutable/Set.scala:41: error: covariant
type A occurs in contravariant position in type
=> scala.collection.parallel.Combiner[A,
scala.collection.parallel.immutable.ParSet[A]] of method parCombiner

Author: smarter

src/library/scala/collection/immutable/Set.scala

@@ -38,7 +38,7 @@ trait Set[A] extends Iterable[A]
   override def toSet[B >: A]: Set[B] = to[({type l[a] = immutable.Set[B]})#l] // for bincompat; remove in dev
 
   override def seq: Set[A] = this
-  protected override def parCombiner = ParSet.newCombiner[A] // if `immutable.SetLike` gets introduced, please move this there!
+  protected[this] override def parCombiner = ParSet.newCombiner[A] // if `immutable.SetLike` gets introduced, please move this there!
 }
 
 /** $factoryInfo