Move gn secondary_source to the build/ directory

build/secondary/third_party/speex/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.
 
 config("speex_config") {
   # Clients expect <speex/speex.h> to be a system header.
   include_dirs = [ "include" ]
 }
 
 static_library("speex") {
@@ skipping 70 matching lines @@
   configs += [ "//build/config/compiler:no_chromium_code" ]
   direct_dependent_configs = [ ":speex_config" ]
 
   if (is_win) {
     cflags = [
       "/wd4244",  # Possible loss of data in conversion.
       "/wd4305",  # Truncation in conversion.
     ]
   }
 }