scons/windows: Fix build with LLVM>=8

Fixes eebe091d
("scons/windows: Enable compute shaders when possible.")
Signed-off-by: Prodea Alexandru-Liviu <liviuprodea@yahoo.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
40 jobs for !2215 with windows-llvm8-9-build-fix in 3 minutes and 28 seconds (queued for 5 seconds)
merge request