dtrace: resolve conversion warnings from SLURP_INT

Resolve build warnings on Windows with the following pattern:
warning C4244: '=': conversion from 'int64_t' to 'int32_t',
possible loss of data

PR-URL: https://github.com/nodejs/node/pull/10143


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>