Fix build failure

Author: natsukagami

library/src/scala/collection/mutable/TreeMap.scala

@@ -201,7 +201,7 @@ sealed class TreeMap[K, V] private (tree: RB.Tree[K, V])(implicit val ordering:
 
     override def get(key: K) = if (isInsideViewBounds(key)) RB.get(tree, key) else None
 
-    override def iterator = if (RB.size(tree) == 0) Iterator.empty else RB.iterator(tree, from, until)
+    override def iterator: Iterator[(K, V)] = if (RB.size(tree) == 0) Iterator.empty else RB.iterator(tree, from, until)
     override def keysIterator: Iterator[K] = if (RB.size(tree) == 0) Iterator.empty else RB.keysIterator(tree, from, until)
     override def valuesIterator: Iterator[V] = if (RB.size(tree) == 0) Iterator.empty else RB.valuesIterator(tree, from, until)
     override def keysIteratorFrom(start: K) = if (RB.size(tree) == 0) Iterator.empty else RB.keysIterator(tree, pickLowerBound(Some(start)), until)