[bazel] Use same expat_config.h as GN rules

This is structured the same way as our configurations of
Freetype and Harfbuzz.

This also makes sure we run the CheckGeneratedFiles job
if DEPS changes, as bazel/deps.bzl is generated from
that file.

Change-Id: Iad5bbbec01434f5ab2ab18efe9821dfbc7830a57
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/610657
Reviewed-by: Ben Wagner <bungeman@google.com>
8 files changed