GN: Fix 32-bit Mac library error

The error hasn't been noticed since we don't really do
(or support) Mac 32-bit builds.

BUG=webrtc:5949
NOTRY=True

Review-Url: https://codereview.webrtc.org/2065583002
Cr-Commit-Position: refs/heads/master@{#13111}

webrtc/base/BUILD.gn

@@ -61,7 +61,7 @@ config("rtc_base_all_dependent_config") {
       "SystemConfiguration.framework",
     ]
     if (current_cpu == "x86") {
-      libs = [ "Carbon.framework" ]
+      libs += [ "Carbon.framework" ]
     }
   }
 }