use WXDLLIMPEXP_FWD instead of WXDLLIMPEXP to avoid gcc4 warnings

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@49804 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/collpaneg.h

@@ -13,8 +13,8 @@
 #define _WX_COLLAPSABLE_PANE_H_GENERIC_
 
 // forward declared
-class WXDLLEXPORT wxButton;
-class WXDLLEXPORT wxStaticLine;
+class WXDLLIMPEXP_FWD_CORE wxButton;
+class WXDLLIMPEXP_FWD_CORE wxStaticLine;
 
 // class name
 extern WXDLLIMPEXP_DATA_CORE(const wxChar) wxCollapsiblePaneNameStr[];