Similar to V8, we provide the include/ headers through a "base" target.

Without this change, `gn check` complains about including cppgc headers
in Blink.

Bug: chromium:1056170
Change-Id: I09ad943cdfedae8c83ab7f957efba796637b8d48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2617087Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71979}