Fixes for C++ compilers

```
/usr/include/libmowgli-2/ext/json.h:45:14: error: use of enum ‘_mowgli_json_tag_t’ without previous declaration
/usr/include/libmowgli-2/ext/json.h:45:50: error: invalid type in declaration before ‘;’ token`
```

```
/usr/include/libmowgli-2/ext/json-inline.h: In function ‘mowgli_json_t* mowgli_json_object_retrieve(mowgli_json_t*, const char*)’:
/usr/include/libmowgli-2/ext/json-inline.h:83:62: error: invalid conversion from ‘void*’ to ‘mowgli_json_t* {aka _mowgli_json_t*}’ [-fpermissive]
/usr/include/libmowgli-2/ext/json-inline.h: In function ‘mowgli_json_t* mowgli_json_object_delete(mowgli_json_t*, const char*)’:
/usr/include/libmowgli-2/ext/json-inline.h:88:60: error: invalid conversion from ‘void*’ to ‘mowgli_json_t* {aka _mowgli_json_t*}’ [-fpermissive]`
```