Fix build with unbundled harfbuzz

210a7ea994ee4b4548894d190914dbd611316ed6 introduces a direct import of
harfbuzz headers from third_party; this is inconsistent with the main harfbuzz
include, inconsistent with everywhere else that `hb-ot.h` is included, and
completely breaks builds where harfbuzz is unbundled.

Bug: https://crbug.com/446344035
Change-Id: I476f9e971d15ab4be8d8ab2d19b9047791e6c217
Signed-off-by: Matt Jolly <kangie@gentoo.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965266
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519197}