Fix compiler warning in NativeBufferImageSiblingAndroid.h. BUG=angleproject:4026 Change-Id: If66f21c4d5693213611b05fa71ba1da328ea612c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1866078 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>