Move mojo::{Wait,WaitMany}() to (the newly-added) mojo/public/cpp/system/wait.h.

mojo/public/cpp/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")
 
 mojo_sdk_source_set("system") {
   sources = [
     "buffer.h",
     "core.h",
     "data_pipe.h",
     "handle.h",
     "macros.h",
     "message_pipe.h",
     "time.h",
+    "wait.h",
   ]
 
   mojo_sdk_public_deps = [ "mojo/public/c/system" ]
 }