Change the canonical way to include the C macros.h to <mojo/macros.h>.

mojo/public/c/system/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("../../mojo_sdk.gni")
 
 # This target merely declares the Mojo system API's types, functions, etc. To
 # use the functions, you must also depend on a specific implementation (e.g.,
 # ../../platform/native:system).
 mojo_sdk_source_set("system") {
   sources = [
     "buffer.h",
     "data_pipe.h",
     "handle.h",
     "macros.h",
     "message_pipe.h",
     "result.h",
     "time.h",
     "wait.h",
     "wait_set.h",
   ]
+
+  mojo_sdk_public_deps = [ "mojo/public/c:common" ]
 }