CPP-993 Build failures due to std::underlying_type (#560)

src/atomic/atomic_std.hpp

diff --git a/src/atomic/atomic_std.hpp b/src/atomic/atomic_std.hpp
index 1a23cc1..08418a9 100644
--- a/src/atomic/atomic_std.hpp
+++ b/src/atomic/atomic_std.hpp
@@ -18,6 +18,7 @@
 #define DATASTAX_INTERNAL_ATOMIC_STD_HPP
 
 #include <atomic>
+#include <type_traits>
 
 namespace datastax { namespace internal {