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/bench.gypi b/gyp/bench.gypi
index a637202..e102a97 100644
--- a/gyp/bench.gypi
+++ b/gyp/bench.gypi
@@ -6,6 +6,7 @@
   'include_dirs': [
     '../bench/subset',
     '../bench',
+    '../include/private',
     '../src/core',
     '../src/effects',
     '../src/gpu',