Make it possible to forward declare struct json_t
Make it possible to forward declare struct json_t

This allows e.g. to use json_t in a header file that doesn't include
jansson.h:

    struct json_t;
    typedef struct json_t json_t;

    json_t *foo(json_t *bar);

firepick-delta/FireSight,bstarynk/jansson,simonfojtu/FireSight,attie/jansson,bstarynk/jansson,attie/jansson,chrullrich/jansson,firepick1/jansson,markalanj/jansson,markalanj/jansson,Mephistophiles/jansson,AmesianX/jansson,OlehKulykov/jansson,firepick1/jansson,Mephistophiles/jansson,wirebirdlabs/featherweight-jansson,chrullrich/jansson,firepick1/FireSight,rogerz/jansson,firepick1/FireSight,OlehKulykov/jansson,Vorne/jansson,AmesianX/jansson,liu3tao/jansson,liu3tao/jansson,npmccallum/jansson,firepick1/FireSight,simonfojtu/FireSight,simonfojtu/FireSight,slackner/jansson,wirebirdlabs/featherweight-jansson,Vorne/jansson,rogerz/jansson,firepick-delta/FireSight,rogerz/jansson,akheron/jansson,akheron/jansson,markalanj/jansson,firepick1/jansson,npmccallum/jansson,rogerz/jansson,firepick-delta/FireSight,bstarynk/jansson,slackner/jansson,akheron/jansson