src: fix compiler warning in node_os

Currently the following compiler warnings is generated:
../src/node_os.cc:167:24:
warning: comparison of integers of different signs: 'size_t'
(aka 'unsigned long') and 'int' [-Wsign-compare]
  for (size_t i = 0; i < count; i++) {
                     ~ ^ ~~~~~
1 warning generated.

This commit changes the type of i to int.

PR-URL: https://github.com/nodejs/node/pull/24356


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>