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

gyp/tests.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # Common gypi for unit tests.
 {
   'include_dirs': [
+    '../include/private',
     '../src/codec',
     '../src/core',
     '../src/effects',
     '../src/image',
     '../src/lazy',
     '../src/images',
     '../src/pathops',
     '../src/pdf',
     '../src/ports',
     '../src/pipe/utils',
@@ skipping 39 matching lines @@
     '../tests/SkpSkGrTest.cpp',
     '../tests/skia_test.cpp',
     '../tests/PathOpsAngleIdeas.cpp',
     '../tests/PathOpsBattles.cpp',
     '../tests/PathOpsCubicLineIntersectionIdeas.cpp',
     '../tests/PathOpsDebug.cpp',
     '../tests/PathOpsOpLoopThreadedTest.cpp',
     '../tests/PathOpsSkpClipTest.cpp',
   ],
 }