src: fix compiler warning in node_worker.cc

Currently, when configuring --without-ssl the following compiler warning
is generated:
../src/node_worker.cc:192:10:
warning: unused variable 'inspector_started' [-Wunused-variable]
    bool inspector_started = false;
         ^
1 warning generated.

This commit adds a macro guard to the variable to avoid the warning.

PR-URL: https://github.com/nodejs/node/pull/28198


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>