Add fallback to ns_client_get_type despite unreachable

GCC might fail to compile because it expects a return after UNREACHABLE.
It should ideally just work anyway since UNREACHABLE is either a
noreturn or UB (__builtin_unreachable / C23 unreachable).

Either way, it should be optimized almost always so the fallback is
free or basically free anyway when it isn't optimized out.

lib/ns/client.c

@@ -155,6 +155,8 @@ ns_client_transport_type(const ns_client_t *client) {
 	case isc_nm_nonesocket:
 		UNREACHABLE();
 	}
+
+	return DNS_TRANSPORT_UDP;
 }
 
 void