Add the baud rate of 256k for Windows (closes #603)

src/modbus-rtu.c

@@ -496,6 +496,9 @@ static int _modbus_rtu_connect(modbus_t *ctx)
     case 250000:
         dcb.BaudRate = 250000;
         break;
+    case 256000:
+        dcb.BaudRate = 256000;
+        break;
     case 460800:
         dcb.BaudRate = 460800;
         break;