Enable deprecation warnings on deps of jsoncpp on Fuchsia

Follow-up to https://chromium-review.googlesource.com/c/chromium/src/+/4066799.
Things already build fine on Fuchsia, so enable
the warning there too.

Also invert the condition from opt-out to opt-in, to
make it clearer on which platforms the warning is still
disabled for now: Android, ChromeOS, Linux, Windows.

Bug: 983223
Change-Id: Ib48058d1e4e6a5408be4fe1cfe4bc2992a1949b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4067159
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1077477}
NOKEYCHECK=True
GitOrigin-RevId: 7d42410c8d02f658a8ac8e5b4d2f2b1bc52ded29