json-c: Don't build unneeded apps to unblock builds with CMake 4+

json-c: Don't build unneeded apps to unblock builds with CMake 4+

Disable build of the apps, as we don't seem to package them (upstream says they
aren't ready, see [0]). They are the only part of the code that doesn't build
yet with CMake 4+. One alternative would be to apply [1], but the PR hasn't been
merged yet by upstream, so we keep it simple.

[0]: https://github.com/json-c/json-c/blob/7cee5237dc6c0831e3f9dc490394eaea44636861/apps/CMakeLists.txt#L119-L121
[1]: https://github.com/json-c/json-c/pull/888

Signed-off-by: Moritz Haase <Moritz.Haase@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>