build: restore libplatform headers in distribution

Headers considered non-essential were removed in #37570, however the
libplatform API is actualy needed (and used) by external software
initiaing the v8 engine, see for example:
https://chromium.googlesource.com/v8/v8/+/refs/heads/main/samples/hello-world.cc

PR-URL: https://github.com/nodejs/node/pull/39288


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>