Fix unused variable warning. Refs #8851.

Bit annoying that `boost::ignore_unused_variable_warning` is in
`<boost/concept_check.hpp>`.

[SVN r85244]

include/boost/unordered/detail/util.hpp

@@ -28,6 +28,8 @@ namespace boost { namespace unordered { namespace detail {
     struct move_tag {};
     struct empty_emplace {};
 
+    template <class T> inline void ignore_unused_variable_warning(T const&) {}
+
     ////////////////////////////////////////////////////////////////////////////
     // iterator SFINAE