modbus: fix compiler warnings about alignment

modbus: fix compiler warnings about alignment

app-layer-modbus.c:1226:39: warning: taking address of packed member 'transactionId' of class or structure 'ModbusHeader_' may result in an unaligned pointer value [-Waddress-of-packed-member]
    if (ModbusExtractUint16(modbus, &(header->transactionId), input, input_len, &offset)    ||
                                      ^~~~~~~~~~~~~~~~~~~~~
app-layer-modbus.c:1228:39: warning: taking address of packed member 'protocolId' of class or structure 'ModbusHeader_' may result in an unaligned pointer value [-Waddress-of-packed-member]
        ModbusExtractUint16(modbus, &(header->protocolId), input, input_len, &offset)       ||
                                      ^~~~~~~~~~~~~~~~~~
app-layer-modbus.c:1230:39: warning: taking address of packed member 'length' of class or structure 'ModbusHeader_' may result in an unaligned pointer value [-Waddress-of-packed-member]
        ModbusExtractUint16(modbus, &(header->length), input, input_len, &offset)           ||
                                      ^~~~~~~~~~~~~~
3 warnings generated.

Bug #2088