Improve the performance of Boost.Function's swap. Thanks to Niels Dekker for the original patch. Fixes #1910

[SVN r48615]

include/boost/function/function_fwd.hpp

@@ -26,6 +26,8 @@ namespace boost { namespace python { namespace objects {
 #endif
 
 namespace boost {
+  class bad_function_call;
+
 #if !defined(BOOST_FUNCTION_NO_FUNCTION_TYPE_SYNTAX)
   // Preferred syntax
   template<typename Signature> class function;