Work around a local type inference limitation (fixes the build)

This is probably an instance of scala/bug#3528

Author: lrytz

library/src/scala/collection/immutable/SeqMap.scala

@@ -171,7 +171,7 @@ object SeqMap extends MapFactory[SeqMap] {
       else {
         // Directly create the elements for performance reasons
         val fields = Vector(key1, key2, key3, key4, key)
-        val underlying = if (useBaseline)
+        val underlying: Map[K, (Int, V1)] = if (useBaseline)
           HashMap(
             (key1, (0, value1)),
             (key2, (1, value2)),