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

gyp/xps.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': 'xps',
       'product_name': 'skia_xps',
       'type': 'static_library',
       'standalone_static_library': 1,
       'conditions': [
         [ 'skia_os == "win"', {
           'defines': [ 'SK_XPS_USE_DETERMINISTIC_IDS', ],
           'dependencies': [
             'skia_lib.gyp:skia_lib',
           ],
           'include_dirs': [
             '../include/device/xps',
+            '../include/private',
             '../include/utils/win',
             '../src/core', # needed to get SkGlyphCache.h
             '../src/utils', # needed to get SkBitSet.h
           ],
           'sources': [
             '../include/device/xps/SkConstexprMath.h',
             '../include/device/xps/SkXPSDevice.h',
             '../src/device/xps/SkXPSDevice.cpp',
             '../src/doc/SkDocument_XPS.cpp',
           ],
@@ skipping 11 matching lines @@
             ],
           },
         },{ #else if 'skia_os != "win"'
           'sources': [ '../src/doc/SkDocument_XPS_None.cpp', ],
           'dependencies': [ 'skia_lib.gyp:skia_lib', ],
         }],
       ],
     },
   ],
 }