Do not include boost/mp11.hpp if already included, to allow use on Compiler Explorer

include/boost/variant2/variant.hpp

@@ -8,7 +8,9 @@
 //  See accompanying file LICENSE_1_0.txt or copy at
 //  http://www.boost.org/LICENSE_1_0.txt
 
+#ifndef BOOST_MP11_HPP_INCLUDED
 #include <boost/mp11.hpp>
+#endif
 #include <boost/config.hpp>
 #include <boost/detail/workaround.hpp>
 #include <cstddef>