Fix compat with boost/integer

[georgthegreat]

Fixes compatibility regression introduced in 470537f.

[FireBurn]

Thanks this fixes an error I was seeing:

error: implicit conversion loses integer precision: 'value_type' (aka 'unsigned long') to 'u32_le' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
   72 |     message.header.crc = crc.checksum();
      |                        ~ ~~~~^~~~~~~~~~

[georgthegreat]

@pdimov, why not merge this (simpler) variant instead of
bd22b63