build: restore libplatform headers in distribution

I maintain libnode/v8 bindings for the R programming language, which depend on libnode on most Linux distro's, e.g. r-cran-v8 in Debian/Ubuntu and R-V8 in Fedora.

Headers considered non-essential were removed in #37570, however the libplatform API is actually needed by external software initiating the v8 engine. See for example the upstream v8 hello world example: https://chromium.googlesource.com/v8/v8/+/refs/heads/main/samples/hello-world.cc

I have also asked upstream v8 if the API in question could maybe be moved to the top level v8-* headers, but they have not responded.

I hope it can be reconsidered to ship these headers, otherwise it will no longer be possible for us to build bindings for libnode, which would be very unfortunate, because they are widely used.