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

Updates #31920

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

src/internal/syscall/windows/sysdll/sysdll.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
+
 // Package sysdll is an internal leaf package that records and reports
 // which Windows DLL names are used by Go itself. These DLLs are then
 // only loaded from the System32 directory. See Issue 14959.