syscall: make mksyscall_windows.go to work even when output does not uses unsafe package

Fixes #9900

Change-Id: I5dd401e8d2040e84ccb97c2fe9f5c5a28095b538
Reviewed-on: https://go-review.googlesource.com/5005Reviewed-by: Minux Ma <minux@golang.org>

src/syscall/mksyscall_windows.go

@@ -736,6 +736,8 @@ package {{packagename}}
 import "unsafe"{{if syscalldot}}
 import "syscall"{{end}}
 
+var _ unsafe.Pointer
+
 var (
 {{template "dlls" .}}
 {{template "funcnames" .}})

src/syscall/zsyscall_windows.go

@@ -4,6 +4,8 @@ package syscall
 
 import "unsafe"
 
+var _ unsafe.Pointer
+
 var (
 	modkernel32 = NewLazyDLL("kernel32.dll")
 	modadvapi32 = NewLazyDLL("advapi32.dll")