build: linux -> windows cross compilation failing (only if building Dawn from source)

`fatal error: 'ObjIdl.h' file not found` etc. because Linux is typically case-sensitive filesystem. Doesn't affect macOS->windows or windows natively because both are case-insensitive.

At least these instances are problematic:

https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/hexops/+%3CWindows.h%3E+OR+%3CObjIdl.h%3E++fork:yes&patternType=standard&case=yes