Fix warning injected into user code by @chiselName (#2500)

In Scala 2.13, Auto-application to `()` is deprecated. Any nullary
method (ie. a method with no arguments) that is defined with () must now
be called with (). @chiselName used to inject a case of this warning
into user code which would then cause a warning on the @chiselName macro
that the user has no way to fix.

macros/src/main/scala/chisel3/internal/naming/NamingAnnotations.scala

@@ -125,7 +125,7 @@ class NamingTransforms(val c: Context) {
     q"""
     val $contextVar = $globalNamingStack.pushContext()
     ..$transformedBody
-    if($globalNamingStack.length == 1){
+    if($globalNamingStack.length() == 1){
       $contextVar.namePrefix("")
     }
     $globalNamingStack.popReturnContext(this, $contextVar)