backport the WXDLLIMPEXP_FWD_XXX macros to 2.8 (patches 1822143 and 1822146)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@49563 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/snglinst.h

@@ -55,7 +55,7 @@ private:
     void Init() { m_impl = NULL; }
 
     // the implementation details (platform specific)
-    class WXDLLIMPEXP_BASE wxSingleInstanceCheckerImpl *m_impl;
+    class WXDLLIMPEXP_FWD_BASE wxSingleInstanceCheckerImpl *m_impl;
 
     DECLARE_NO_COPY_CLASS(wxSingleInstanceChecker)
 };