json: include json.h accordingly to json-c CFLAGS

We use $(pkg-config --cflags json-c) but we include <json/json.h>. The
proper way to include the json-c header is to include <json.h>.

That absolute include causes errors while compiling with json-c-0.12
where the base include directory was changed  to "json-c".

https://github.com/json-c/json-c/blob/master/ChangeLog

Signed-off-by: Jakub Filak <jfilak@redhat.com>