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);

src/jansson.h

@@ -47,7 +47,7 @@ typedef enum {
     JSON_NULL
 } json_type;
 
-typedef struct {
+typedef struct json_t {
     json_type type;
     size_t refcount;
 } json_t;