openxr_platform.h should include unknwn.h for IUnknown on WIN32

[amalon]

Issue #237 was worked around for the SDK build, but the same issue still affects users of the OpenXR headers. unknwn.h needs to be included before openxr_platform.h for WIN32 platform to avoid build errors reported with MSVC 2019 (16.11.18), with OpenXR 1.0.24:
https://sourceforge.net/p/flightgear/mailman/message/37697116/
Visual studio community 2022 doesn't seem affected.

This is wrong. OpenXR apps shouldn't be expected to anticipate includes that might be required on windows for extensions they don't use or for future extensions they may not know about. There doesn't seem to be any indication in the XR spec that I can find that other includes are required prior to openxr_platform.h.

openxr_platform.h needs fixing to include necessary headers itself, which are reportedly marked up in the XML but not being written out.

[rpavlik-bot]

An issue (number 1817) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#1817 ), to facilitate working group processes.

This GitHub issue will continue to be the main site of discussion.