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
 }

wgengine/router_openbsd.go

@@ -155,7 +155,7 @@ func (r *openbsdRouter) SetRoutes(rs RouteSettings) error {
 	return errq
 }
 
-func (r *openbsdRouter) Close() {
+func (r *openbsdRouter) Close() error {
 	out, err := cmd("ifconfig", r.tunname, "down").CombinedOutput()
 	if err != nil {
 		r.logf("running ifconfig failed: %v\n%s", err, out)
@@ -166,6 +166,8 @@ func (r *openbsdRouter) Close() {
 	}
 
 	// TODO(mbaillie): wipe routes
+
+	return nil
 }
 
 // TODO(mbaillie): these are no-ops for now. They could re-use the Linux funcs

wgengine/router_windows.go

@@ -51,8 +51,9 @@ func (r *winRouter) SetRoutes(rs RouteSettings) error {
 	return nil
 }
 
-func (r *winRouter) Close() {
+func (r *winRouter) Close() error {
 	if r.routeChangeCallback != nil {
 		r.routeChangeCallback.Unregister()
 	}
+	return nil
 }