util/osdiag, util/winutil: expose Windows policy key

The Windows base registry key is already exported but the policy key was
not. util/osdiag currently replicates the string rather than the
preferred approach of reusing the constant.

Updates #cleanup

Change-Id: I6c1c45337896c744059b85643da2364fb3f232f2
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

util/winutil/winutil_notwindows.go

@@ -13,6 +13,7 @@ import (
 )
 
 const regBase = ``
+const regPolicyBase = ``
 
 var ErrNoValue = errors.New("no value because registry is unavailable on this OS")