rdrand: silence a type conversion warning under clang

rand/rdrand.cpp

@@ -40,5 +40,9 @@ rdrand::operator() ()
     if (!__builtin_expect(success, 1))
         throw std::runtime_error ("no value available for rdrand");
 
-    return res;
+    // As long as the u64 we just grabbed is of a type we can truncate then
+    // we'll do just that and return it.
+    static_assert (sizeof (result_type) <= sizeof (res));
+    static_assert (std::is_signed_v<result_type> == std::is_signed_v<decltype(res)>);
+    return static_cast<result_type> (res);
 }