GN: Fix 32-bit Mac library error

webrtc/base/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/crypto.gni")
 import("//build/config/ui.gni")
@@ skipping 43 matching lines @@
   }
   if (is_mac) {
     libs = [
       "Cocoa.framework",
       "Foundation.framework",
       "IOKit.framework",
       "Security.framework",
       "SystemConfiguration.framework",
     ]
     if (current_cpu == "x86") {
-      libs = [ "Carbon.framework" ]
+      libs += [ "Carbon.framework" ]
     }
   }
 }
 
 if (is_linux && !build_with_chromium) {
   # Provides the same functionality as the //crypto:platform target, which
   # WebRTC cannot use as we don't sync src/crypto from Chromium.
   group("linux_system_ssl") {
     deps = [
       "//third_party/boringssl",
@@ skipping 594 matching lines @@
     ]
     deps = [
       ":rtc_base",
       "../test:field_trial",
     ]
     public_deps = [
       "//testing/gtest",
     ]
   }
 }