Fix gn build.

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.
 
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
 v8_compress_startup_data = "off"
 v8_deprecation_warnings = false
 v8_enable_disassembler = false
 v8_enable_gdbjit = false
@@ skipping 878 matching lines @@
     "src/scopeinfo.h",
     "src/scopes.cc",
     "src/scopes.h",
     "src/serialize.cc",
     "src/serialize.h",
     "src/small-pointer-list.h",
     "src/smart-pointers.h",
     "src/snapshot-source-sink.cc",
     "src/snapshot-source-sink.h",
     "src/snapshot.h",
+    "src/string-builder.cc",
     "src/string-builder.h",
-    "src/string-builder.c",
     "src/string-search.cc",
     "src/string-search.h",
     "src/string-stream.cc",
     "src/string-stream.h",
     "src/strtod.cc",
     "src/strtod.h",
     "src/token.cc",
     "src/token.h",
     "src/transitions-inl.h",
     "src/transitions.cc",
@@ skipping 546 matching lines @@
     deps = [
       ":v8_base",
       ":v8_nosnapshot",
     ]
   }
 
   direct_dependent_configs = [ ":external_config" ]
 }
 
 }