core/lldp: avoid default switch case in lldp_neighbor_to_variant()

Explicitly check for LLDP_ATTR_TYPE_NONE. That's the only one we expect,
and the compiler can warn about missing switch cases for enums.

src/devices/nm-lldp-listener.c

@@ -813,7 +813,7 @@ lldp_neighbor_to_variant (LldpNeighbor *neigh)
 			                       g_variant_builder_end (&builder2));
 			break;
 		}
-		default:
+		case LLDP_ATTR_TYPE_NONE:
 			break;
 		}
 	}