yajl is not optional

If I don't install yajl headers, I get the following error:

```
/home/avatar/tmp/oyranos/openicc/oyjl/oyjl_yajl.c: In function ‘oyjlTreeParse’:
/home/avatar/tmp/oyranos/openicc/oyjl/oyjl_yajl.c:471:3: error: ‘handle_string’ undeclared (first use in this function); did you mean ‘handle_null’?
   handle_string,
   ^~~~~~~~~~~~~
```