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

gyp/effects.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 for effects
 {
   'targets': [
     {
       'target_name': 'effects',
       'product_name': 'skia_effects',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'core.gyp:*',
         'images.gyp:*',
         'utils.gyp:utils',
       ],
       'includes': [
         'effects.gypi',
       ],
       'include_dirs': [
         '../include/effects',
+        '../include/private',
         '../src/effects',
         '../src/opts',
         '../src/core',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/effects',
         ],
       },
       'sources': [
         'effects.gypi', # Makes the gypi appear in IDEs (but does not modify the build).
       ],
       'conditions': [
         ['skia_gpu == 1', {
           'include_dirs': [
             '../include/gpu',
             '../src/gpu',
           ],
         }],
       ],
     },
   ],
 }