add missing include for SkIsPow2

Change-Id: I2df438747d12d5b860bb11083676f0e8e43b931c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/470016
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed