Added hash value for boost::basic_string_view for older version of boost.

Author: redboltz

include/mqtt/string_view.hpp

@@ -40,6 +40,16 @@ using string_view = boost::string_view;
 template<class CharT, class Traits = std::char_traits<CharT> >
 using basic_string_view = boost::basic_string_view<CharT, Traits>;
 
+#if BOOST_VERSION < 106900
+
+#include <boost/container_hash/hash_fwd.hpp>
+
+template <class charT, class traits>
+std::size_t hash_value(basic_string_view<charT, traits> s) {
+    return boost::hash_range(s.begin(), s.end());
+}
+#endif // BOOST_VERSION < 106900
+
 } // namespace MQTT_NS
 
 #else  // BOOST_VERSION >= 106100