tweak SkOnce.h header to work around issues with std::atomic::compare_exchange_strong

BUG=skia:

Change-Id: Id29c47cb4bec6a6d70d6514d18fa4ab9bd31b1e2
Reviewed-on: https://skia-review.googlesource.com/6200
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed