IGNORE_WARNINGS flag in extension build in CMake is not handled correctly

[Mytherin]

There is a typo in the definition:

set(INGORE_WARNINGS OFF)
build_loadable_extension(json ${JSON_EXTENSION_FILES} ${IGNORE_WARNINGS})

This causes the flag to be empty, and because CMake is CMake it does not crash on an unknown variable but instead passes in an empty string. It is also passed in as a source file (as the build_loadable_extension is a varargs function that uses everything after the extension name as a source file), so fixing the typo breaks the build as well.

[omo]

(Unsolicited triage) Can be closed now by #4866 ? @pdet