remove reference to std::unary_function

It is not part of c++17 and this code is supposed to build as c++17.

See https://en.cppreference.com/w/cpp/utility/functional/unary_function.

include/irrUString.h

@@ -3678,7 +3678,7 @@ namespace unicode
 
 //! Hashing algorithm for hashing a ustring.  Used for things like unordered_maps.
 //! Algorithm taken from std::hash<std::string>.
-class hash : public std::unary_function<core::ustring, size_t>
+class hash
 {
 	public:
 		size_t operator()(const core::ustring& s) const