Add dllexport to common_config_file_iterator.

As reported in #6797, there are linker errors otherwise. I
do not know why regression tests, which test dll linking
explicitly, do not catch this.

include/boost/program_options/detail/config_file.hpp

@@ -62,7 +62,7 @@ namespace boost { namespace program_options { namespace detail {
          TODO: maybe, we should just accept a pointer to options_description
          class.
      */    
-    class common_config_file_iterator 
+    class BOOST_PROGRAM_OPTIONS_DECL common_config_file_iterator
         : public eof_iterator<common_config_file_iterator, option>
     {
     public: