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

gyp/ktx.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.
 {
   'variables': {
     'skia_warnings_as_errors': 0,
   },
   'targets': [
   {
     'target_name': 'libSkKTX',
     'type': 'static_library',
     'include_dirs' : [
       '../third_party/ktx',
       '../include/gpu',
+      '../include/private',
       '../src/core',
       '../src/gpu',
       '../src/utils',
     ],
     'sources': [
       '../third_party/ktx/ktx.cpp',
     ],
     'dependencies': [
       'core.gyp:*',
       'etc1.gyp:libetc1',
     ],
     'direct_dependent_settings': {
       'include_dirs': [
         '../third_party/ktx',
       ],
     },
   }],
 }