Add copyright headers to remaining gyp files.

gyp/xml.gyp

+# Copyright 2015 Google Inc.
+#
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
 {
   'targets': [
     {
       'target_name': 'xml',
       'product_name': 'skia_xml',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'skia_lib.gyp:skia_lib',
       ],
@@ skipping 18 matching lines @@
           '../src/xml/SkXMLPullParser.cpp', #if 0 around class decl in header
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/xml',
         ],
       },
     },
   ],
 }