Fix iOS builders.

Gyp... sigh.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4787

Change-Id: I5488bbc6a8e267dc42d1c716cf0f879b662af171
Reviewed-on: https://skia-review.googlesource.com/4787
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gyp/tools.gyp b/gyp/tools.gyp
index b4ce755..42638fd 100644
--- a/gyp/tools.gyp
+++ b/gyp/tools.gyp
@@ -404,6 +404,7 @@
         '../tools/picture_utils.h',
       ],
       'include_dirs': [
+          '../include/private',
           '../src/core/',
       ],
       'dependencies': [