Add BUILD.gn for ios/third_party/fishhook.

ios/third_party/fishhook/BUILD.gn

+# 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.
+
+config("fishhook_config") {
+  include_dirs = [ "src" ]
+  visibility = [ ":fishhook" ]
+}
+
+source_set("fishhook") {
+  testonly = true
+  sources = [
+    "src/fishhook.c",
+    "src/fishhook.h",
+  ]
+
+  public_configs = [ ":fishhook_config" ]
+  configs += [ ":fishhook_config" ]
+}