library_config: fix Cflags include

[yangfl]

No description provided.

[FSMaxB]

So about that: Since there is no include path added to the pkgconfig file, the canonical way to include cJSON if it wasn't copy-pasted into your source code is #include <cjson/cJSON.h>.

Adding the additional include directory would also allow #include <cJSON.h>.

I'm not very opiniated about this other than that <cjson/cJSON> still has to work, but I would like to get some feedback by other users first, maybe there are some implications that I didn't think of.

Also note that not only the pkgconfig file needs to be changed but also cJSONConfig.cmake.in.

[yangfl]

OK, I've seen and I'm not against <cjson/cJSON>
But if you decide to use <cjson/cJSON>, could you mention it somewhere in the documentation?

[FSMaxB]

I added a section to the documentation.