Move gn secondary_source to the build/ directory

build/secondary/third_party/icu/BUILD.gn

 # Copyright (c) 2012 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.
 
 # Meta target that includes both icuuc and icui18n. Most targets want both.
 # You can depend on the individually if you need to.
 group("icu") {
   deps = [
     ":icui18n",
     ":icuuc",
@@ skipping 424 matching lines @@
       # the icudt46_dat symbol.
       "android/icudt46l_dat.S",
       "linux/icudt46l_dat.S",
       "mac/icudt46l_dat.S",
     ]
     defines = [ "U_HIDE_DATA_SYMBOL" ]
 
     # TODO(brettw) support use_system_icu and icu_use_data_file_flag.
   }
 }