droppie: fix missing systemd wantedBy

without this, the systemd service does not start after a reboot

hosts/droppie/droppie.nix

@@ -37,6 +37,7 @@ in
         Restart = "always";
         RestartSec = "5s";
       };
+      wantedBy = [ "multi-user.target" ];
     };
   };
 }