build: Fix wrong OS defines

OS check should use BUILDFLAG.

Bug: 12340436
Change-Id: I25718ad8ab09ea098bc82039bfa5f4d48a8f78ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3601298


Reviewed-by: Nico Weber <thakis@chromium.org>
Owners-Override: Nico Weber <thakis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#995413}