headers/openvr.h: Allow user to specify ABI type

[aeikum]

The use for this is so we can load the native Linux openvr library when building DXVK as a native Linux library. Winelib builds define _WIN32, but we want to use the Unix ABI for openvr, so let's let users request a specific ABI and fallback on the old logic if one is not requested.

(I think this can't be merged directly, but hopefully you can apply this diff to whatever file generates openvr.h easily enough.)