Apply gcc's "const" attribute to the json_c_object_sizeof() function as an optimizer hint. Also, rename that function from json_object_sizeof().

json_object.c

@@ -509,7 +509,7 @@ int json_object_object_length(const struct json_object *jso)
 	return lh_table_length(jso->o.c_object);
 }
 
-size_t json_object_sizeof(void)
+size_t json_c_object_sizeof(void)
 {
 	return sizeof(struct json_object);
 }