Cease dependence on SmartPtr

by forward declaring `boost::shared_ptr`

include/boost/type_index/runtime_cast/boost_shared_ptr_cast.hpp

@@ -15,12 +15,15 @@
 
 #include <boost/type_index/runtime_cast/detail/runtime_cast_impl.hpp>
 #include <boost/type_traits/is_base_and_derived.hpp>
-#include <boost/smart_ptr/shared_ptr.hpp>
 
 #ifdef BOOST_HAS_PRAGMA_ONCE
 # pragma once
 #endif
 
+namespace boost {
+    template<class T> class shared_ptr;
+}
+
 namespace boost { namespace typeindex {
 
 /// \brief Creates a new instance of std::shared_ptr whose stored pointer is obtained from u's