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/svg.gyp b/gyp/svg.gyp
index df0c45c..ddcbb93 100644
--- a/gyp/svg.gyp
+++ b/gyp/svg.gyp
@@ -14,6 +14,7 @@
         'xml.gyp:*',
       ],
       'include_dirs': [
+        '../include/private',
         '../include/svg',
         '../src/core',
       ],