fix compilation problem with gcc-5

..while we should note at this point that the whole techique
of hijacking std::hash is superfluous now, since the standard libray
does no longer define a static assertion which defeats SFINAE

src/lib/hash-standard.hpp

@@ -72,6 +72,7 @@
 
 
 #include <cstddef>
+#include <utility>
 #include <boost/utility/enable_if.hpp>