microsoft/spirv_to_dxil: Fix non-Windows build.

microsoft/spirv_to_dxil: Fix non-Windows build.

../src/microsoft/spirv_to_dxil/dxil_validation.cpp: In function ‘bool validate_dxil(dxil_spirv_object*)’:
../src/microsoft/spirv_to_dxil/dxil_validation.cpp:129:12: error: ‘stderr’ was not declared in this scope
  129 |    fprintf(stderr, "DXIL validation only available in Windows.\n");
      |            ^~~~~~

Fixes: 37c366e2830 ("microsoft/spirv_to_dxil: Add DXIL validation to spirv2dxil")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13736>