Fix mingw compile.
Fix mingw compile.

Adjust a compile guard around posix_memalign.

Bug: angleproject:2163
Change-Id: I1d2dca24b8a2b8e549df411b1611e9fe0ac9082d
Reviewed-on: https://chromium-review.googlesource.com/920748
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle