compilation fix for g++ 3 (and probably others) after the last commit

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45269 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/stringimpl.h

@@ -239,6 +239,11 @@ public:
                                                                               \
         pointer m_ptr
 
+  // we need to declare const_iterator in wxStringImpl scope, the friend
+  // declaration inside iterator class itself is not enough, or at least not
+  // for g++ 3.4 (g++ 4 is ok)
+  class const_iterator;
+
   class iterator
   {
     WX_DEFINE_STRINGIMPL_ITERATOR(iterator,