Adjust a compile guard around posix_memalign.

Bug: angleproject:2163
Change-Id: I1d2dca24b8a2b8e549df411b1611e9fe0ac9082d
Reviewed-on: https://chromium-review.googlesource.com/920748Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>