Add BUILD.gn for ios/third_party/fishhook.

ios/third_party/fishhook/fishhook.gyp

 # Copyright 2016 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.
 
 {
   'targets': [
     {
+      # GN version: //ios/third_party/fishhook
       'target_name': 'fishhook',
       'type': 'static_library',
       'sources': [
         'src/fishhook.c',
         'src/fishhook.h',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           'src'
         ],
       },
     },
   ],
 }