build: stop distclean from deleting v8 files

Stop `make distclean` from deleting files in the `deps/v8/testing/gmock`
folder, thus avoiding deleting version-controlled files important for
v8.

Fixes: https://github.com/nodejs/node/issues/21163

PR-URL: https://github.com/nodejs/node/pull/21164


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>