Added #pragma once to headers

include/boost/intrusive/pointer_plus_bits.hpp

@@ -13,6 +13,10 @@
 #ifndef BOOST_INTRUSIVE_POINTER_PLUS_BITS_HPP
 #define BOOST_INTRUSIVE_POINTER_PLUS_BITS_HPP
 
+#if defined(_MSC_VER)
+#  pragma once
+#endif
+
 #include <boost/intrusive/detail/config_begin.hpp>
 #include <boost/intrusive/intrusive_fwd.hpp>
 #include <boost/intrusive/detail/mpl.hpp> //ls_zeros