src: guard default_inspector_port

[danbev]

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src