util/os_file: replace broken windows-detection code with detect_os.h

The meson-windows-vs2019 job was going down the `!defined(WIN32)` path,
leading to a broken build once that path contained non-windows code.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5369>