Require an explicit ServiceName annotation for interfaces in C++.

mojo/services/vanadium/security/interfaces/principal.mojom

Index: mojo/services/vanadium/security/interfaces/principal.mojom
diff --git a/mojo/services/vanadium/security/interfaces/principal.mojom b/mojo/services/vanadium/security/interfaces/principal.mojom
index d2493a3f406b386bf725ddc1ccae540998bdaae2..c20f8d3049acd7fd2e22cd4464aba63aa9fa407d 100644
--- a/mojo/services/vanadium/security/interfaces/principal.mojom
+++ b/mojo/services/vanadium/security/interfaces/principal.mojom
@@ -35,6 +35,7 @@ struct User {
 // A service that binds user identities to an application instance running in
 // Mojo. An application instance may have multiple user identities with one of
 // them set as the current identity.
+[ServiceName="vanadium::PrincipalService"]
 interface PrincipalService {
   // Login is called by an application instance (requestor_url/qualifier) that
   // wants to get a new user identity. The service may obtain the user identity