[GN build] Add a BUILD.gn file for //third_party/decklink

third_party/decklink/decklink.gyp

 # 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.
 
+# GN version: //third_party/decklink
 {
   'conditions': [
     ['OS=="mac"', {
       'targets': [
         {
           'target_name': 'decklink',
           'type': 'static_library',
           'sources' : [
             'mac/include/DeckLinkAPIDispatch.cpp',
           ],
         },  # target decklink
       ]
      }],
   ]
 }