add -Iinclude/private anywhere we have -Isrc/core

gyp/flags.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.
 # GYP file to build flag parser
 #
 {
   'targets': [
     {
       'target_name': 'flags',
       'type': 'static_library',
-      'include_dirs': [ '../src/core', ],
+      'include_dirs': [
+          '../include/private',
+          '../src/core',
+      ],
       'sources': [
         '../tools/flags/SkCommandLineFlags.cpp',
         '../tools/flags/SkCommandLineFlags.h',
       ],
       'dependencies': [
         'skia_lib.gyp:skia_lib',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../tools/flags',
@@ skipping 12 matching lines @@
         'flags.gyp:flags',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../tools/flags',
         ],
       }
     },
   ],
 }