src: fix string format mistake for 32 bit node

  warning: format ‘%lx’ expects argument of type ‘long
  unsigned int’, but argument 3 has type ‘unsigned int’ [-Wformat=]
     BIO_printf(bio, "0x%lx", exponent_word);

PR-URL: https://github.com/nodejs/node/pull/10082


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>