Fix the last warning with Visual Studio 2008.

src/modbus.c

@@ -427,7 +427,7 @@ int _modbus_receive_msg(modbus_t *ctx, uint8_t *msg, msg_type_t msg_type)
             case _STEP_META:
                 length_to_read = compute_data_length_after_meta(
                     ctx, msg, msg_type);
-                if ((msg_length + length_to_read) > ctx->backend->max_adu_length) {
+                if ((msg_length + length_to_read) > (int)ctx->backend->max_adu_length) {
                     errno = EMBBADDATA;
                     _error_print(ctx, "too many data");
                     return -1;