|exclude_imports| flag is set in some of the perfetto's proto_library
targets to indicate that we don't need to generate the proto-descriptor
for the protos included in those `x.proto` files. In this CL we use that
flag to conditionally pass `--include_imports` argument to protoc.

This is similar to the CL (https://crrev.com/c/2632759)

Bug: b:236945541
Change-Id: I0689003978096798d1e966ec8485cd6af7237804
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3721616Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Mohit Saini <mohitms@google.com>
Cr-Commit-Position: refs/heads/main@{#81356}