make SkClipStack.h private (in src)

BUG=skia:

Change-Id: I05f1140fe483f4a92093cb9783c6e9f067420d30
Reviewed-on: https://skia-review.googlesource.com/8481
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed