Make Client polymorphic by adding a virtual destructor

PiperOrigin-RevId: 675145888
Change-Id: Iad1821642e73db27c522baf4efe64dad78acbf7d

sandboxed_api/sandbox2/client.h

@@ -43,6 +43,7 @@ class Client {
   static constexpr uint32_t kSandbox2ClientUnotify = 0x0A0B0C03;
 
   explicit Client(Comms* comms);
+  virtual ~Client() = default;
 
   Client(const Client&) = delete;
   Client& operator=(const Client&) = delete;