[json-c] Add include folder following the upstream

[uilianries]

The JSON-C project includes "include/json-c" as include directory path, as documented here: https://github.com/json-c/json-c/blob/master/README.md#linking-to-libjson-c-

Plus, the tests are using the header name directly, without a namespace/folder: https://github.com/json-c/json-c/blob/master/tests/parse_flags.c#L9

This PR is required by #19298

/cc @franramirez688

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[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 2 (b3106d2e193e894f101c37236d924a15bb28356b):

• json-c/0.15:
  All packages built successfully! (All logs)

• json-c/0.16:
  All packages built successfully! (All logs)

• json-c/0.17:
  All packages built successfully! (All logs)

• json-c/0.14:
  All packages built successfully! (All logs)

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Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 2 (b3106d2e193e894f101c37236d924a15bb28356b):

• json-c/0.14:
  All packages built successfully! (All logs)

• json-c/0.17:
  All packages built successfully! (All logs)

• json-c/0.16:
  All packages built successfully! (All logs)

• json-c/0.15:
  All packages built successfully! (All logs)

[github-actions]

Hooks produced the following warnings for commit b3106d2

json-c/0.17@#2cb6eafb8de1fb2b3410c35860d14149

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libjson-c.so' links to system library 'm' but it is not in cpp_info.system_libs.