Convert GN single-item list concat to use lists.

tools/gn/secondary/third_party/expat/BUILD.gn

 # Copyright (c) 2013 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.
 
 if (is_linux) {
   config("expat_config") {
     libs = [ "expat" ]
   }
 
   group("expat") {
@@ skipping 11 matching lines @@
   static_library("expat") {
     sources = [
       "files/lib/expat.h",
       "files/lib/xmlparse.c",
       "files/lib/xmlrole.c",
       "files/lib/xmltok.c",
     ]
 
     defines = [ "_LIB" ]
     if (is_win) {
-      defines += "COMPILED_FROM_DSP"
+      defines += [ "COMPILED_FROM_DSP" ]
     } else {
-      defines += "HAVE_EXPAT_CONFIG_H"
+      defines += [ "HAVE_EXPAT_CONFIG_H" ]
     }
   }
 }