jsoncpp: Disable libcxx module

This is to unblock module build in jsoncpp.

Bug: 428778692
Change-Id: I43366ba435501d8b242d5892016dc1a6146b5e54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6768662
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1490751}
NOKEYCHECK=True
GitOrigin-RevId: 24df863d661ad5edb5074821f7c9d8843169ca1e