Fix cmake not building correctly when WITH_CJSON=OFF

[madhat1]

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  It fails before and after my fix, I don't have cJson installed.

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Fix cmake not building correctly when WITH_CJSON=OFF.
In the main CMakeLists.txt, the cJson discovery process is executed regardless if WITH_CJSON is ON or OFF, this MR fixes that.

This fixes #2026

[ralight]

Thanks very much!

[madhat1]

No prob, my pleasure 😀. Is this fix expected to be included in v2.0.6?

[ralight]

Yes, it's in fixes now which will become 2.0.6 in about a week.

[madhat1]

Awesome, keep up the good work mate!
BTW, the latest support for UDS in lib and broker is a big improvement, kudos for that!