Convert GN single-item list concat to use lists.

build/config/clang/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.
 
 config("find_bad_constructs") {
   cflags = [
     "-Xclang", "-load",
     "-Xclang",
   ]
 
   if (is_mac) {
-    cflags += rebase_path(
+    cflags += [ rebase_path(
       "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib",
-      ".", root_build_dir)
+      ".", root_build_dir) ]
   } else if (is_linux) {
-    cflags += rebase_path(
+    cflags += [ rebase_path(
       "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so",
-      ".", root_build_dir)
+      ".", root_build_dir) ]
   }
 
   cflags += [
     "-Xclang", "-add-plugin",
     "-Xclang", "find-bad-constructs",
   ]
 }