Internal-only visibility for rtc_base:macromagic

Bug: webrtc:9606
Change-Id: I31cca1c6c5784630ab88fc11a823ad2aa3844758
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/186043
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32476}

rtc_base/BUILD.gn

@@ -157,8 +157,6 @@ rtc_library("rtc_base_approved") {
 }
 
 rtc_source_set("macromagic") {
-  # TODO(bugs.webrtc.org/9606): This should not be public.
-  visibility = [ "*" ]
   sources = [
     "arraysize.h",
     "constructor_magic.h",