build: Fix compile failure in backtrace_posix.cc

Fix compile failure in backtrace_posix.c on AIX

Fixes: https://github.com/nodejs/node/issues/7539
PR-URL: https://github.com/nodejs/node/pull/7544


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>