#134 GCC 15.2.1 compilation on Linux fails with errors

[Antonio Bartalesi]

GCC 15.2.1 compilation on Linux fails with errors:

3rdparty/libmodbus/modbus.c:180:5: error: implicit declaration of function ‘busMonitorRawRequestData’
3rdparty/libmodbus/modbus.c:475:5: error: implicit declaration of function ‘busMonitorRawResponseData’

The functions are defined in src/modbusadapter.cpp (C++) but called from the C file modbus.c without a forward declaration.

The fix is to add forward declaration in 3rdparty/libmodbus/modbus.c:

extern void busMonitorRawRequestData(uint8t msg, int msglength);
extern void busMonitorRawResponseData(uint8t msg, int msglength);