refactor: use core/ref.hpp over ref.hpp

The later has been deprecated:
```cpp

// The header file at this path is deprecated;
// use boost/core/ref.hpp instead.

```

test/functional/make_unfused.cpp

@@ -19,11 +19,11 @@
 
 #include <boost/utility/result_of.hpp>
 #include <boost/core/enable_if.hpp>
+#include <boost/core/ref.hpp>
 
 #include <boost/fusion/sequence/intrinsic/empty.hpp>
 #include <boost/fusion/algorithm/iteration/fold.hpp>
 
-#include <boost/ref.hpp>
 
 namespace fusion = boost::fusion;
 namespace mpl = boost::mpl;