wgengine: fix the build on darwin, windows, openbsd.

7e5e3277 changed the Router interface definition, but forgot to
fix up these platform files.

Signed-off-by: David Anderson <dave@natulte.net>

wgengine/router_darwin.go

@@ -32,5 +32,6 @@ func (r *darwinRouter) SetRoutes(rs RouteSettings) error {
 	return nil
 }
 
-func (r *darwinRouter) Close() {
+func (r *darwinRouter) Close() error {
+	return nil
 }