Add unistd.h include to GrVkGpu.cpp on unix for sleep

Fixes Fuchsia build

Change-Id: Ieb3b7092586495f5153ba18d14a94d8dcdab1dbd
Reviewed-on: https://skia-review.googlesource.com/10261
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed