internal/syscall/windows/sysdll: mark package as Windows-only
internal/syscall/windows/sysdll: mark package as Windows-only

Updates #31920

Change-Id: Ie24ed5bab249e2f90d1740f42a8b8d94fd0983f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/176019
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,golang/go,golang/go,CAFxX/go