libvpx.gyp: Fix process_outputs_as_sources usage.

unpack_lib_posix.gypi

 # 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.
 
 # This action takes an archive (.a) file and unpacks it unto object (.o) files.
 # The following input gyp variables are required:
 #   unpack_lib_output_dir, the output directory of extracted object file
 #   unpack_lib_name, the object file to be extracted.
 #   unpack_lib_search_path_list, a list of paths to search for the library.
 #   it must be ['-a', 'path_name1', '-a', 'path_name2'...]
@@ skipping 30 matching lines @@
       'outputs': [
         '<(unpack_lib_output_dir)/<(unpack_lib_name)',
       ],
       'action': [
         '<(DEPTH)/third_party/libvpx/unpack_lib_posix.sh',
         '-d', '<(unpack_lib_output_dir)',
         '-f', '<(unpack_lib_name)',
         '<@(unpack_lib_search_path_list)',
         '<@(ar_cmd)',
       ],
-      'process_output_as_sources': 1,
+      'process_outputs_as_sources': 1,
     },
   ],
 }