flatudp: restore the datagram_size declaration deleted by the previous comment fix

src/source/flatudp/flatudp_signals.hpp

@@ -33,8 +33,8 @@ const char* type_name(Type t);  ///< "uint8" ... "float64"
 bool parse_type(const std::string& s, Type& out);
 
 /// The bytes one datagram must have: the sum of `type_size * count`.
+std::size_t datagram_size(const std::vector<Row>& rows);
 /// True when no row is a vector. (Batching depends on `any_wave`, not on this.)
-/// True when no row is a vector. Such a list can be batched, several datagrams to a frame.
 bool all_scalar(const std::vector<Row>& rows);
 
 /// True when some row is a waveform (`wave`).