src: guard exit label when inspector disabled

Currently, when configured --without-inspector the following warning
will be generated:
../src/node.cc:859:1: warning: unused label 'exit' [-Wunused-label]
exit:

This commit adds a guard to exclude the label when there is no
inspector support.

PR-URL: https://github.com/nodejs/node/pull/26801


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>