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

I'll be moving headers from src/core to include/private, so this guarantees
that anyone who was finding them via -Isrc/core can now find them via
-Iinclude/private.

This is purely mechanical, mostly to preserve my sanity, so it's likely
(harmless) overkill.

Chromium's GYP and GN builds already set -Iinclude/private for Skia builds.

BUG=skia:4126

Review URL: https://codereview.chromium.org/1265443002
diff --git a/gyp/pathops_unittest.gypi b/gyp/pathops_unittest.gypi
index 75e824b..d8eac01 100644
--- a/gyp/pathops_unittest.gypi
+++ b/gyp/pathops_unittest.gypi
@@ -5,6 +5,7 @@
 # Common gypi for pathops unit tests.
 {
   'include_dirs': [
+    '../include/private',
     '../src/core',
     '../src/effects',
     '../src/lazy',
@@ -63,8 +64,8 @@
     '../tests/PathOpsTestCommon.cpp',
     '../tests/PathOpsThreadedCommon.cpp',
     '../tests/PathOpsThreeWayTest.cpp',
-    '../tests/PathOpsTightBoundsTest.cpp', 
-    '../tests/PathOpsTypesTest.cpp', 
+    '../tests/PathOpsTightBoundsTest.cpp',
+    '../tests/PathOpsTypesTest.cpp',
 
     '../tests/PathOpsCubicIntersectionTestData.h',
     '../tests/PathOpsExtendedTest.h',