inspector: add missing virtual destructor

This fixes a resource leak detected by a Coverity scan.

PR-URL: https://github.com/nodejs/node/pull/13198


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Aleksei Koziatinskii <ak239spb@gmail.com>