src: guard default_inspector_port

When configuring and building --without-ssl the following warning is
reported:
../src/node_debug_options.cc:12:11: warning: unused variable
'default_inspector_port' [-Wunused-const-variable]
const int default_inspector_port = 9229;

The commit adds a HAVE_INSPECTOR guard to this constant.

PR-URL: https://github.com/nodejs/node/pull/12303


Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>