add cast for printf

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3020

Change-Id: Ib9277facab87747b3cf9b71585e63067b4be9a58
Reviewed-on: https://skia-review.googlesource.com/3020
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed