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

examples/c/echo_client/BUILD.gn

 # Copyright 2015 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/public/mojo_application.gni")
 
 group("echo_client") {
   deps = [
     ":c_echo_client",
   ]
 }
 
 mojo_native_application("c_echo_client") {
   sources = [
     "echo_client.c",
   ]
 
   deps = [
     "//examples/echo:bindings_c",
+    "//mojo/public/c",
     "//mojo/public/c/system",
     "//mojo/public/interfaces/application:application_c",
   ]
 }