fix warning about non-dllexport'ed base wxDataViewHeaderWindowBase class from MSVC in DLL builds

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@44639 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/datavgen.cpp

@@ -63,7 +63,9 @@ static const int PADDING_TOPBOTTOM = 1;
 
 #define USE_NATIVE_HEADER_WINDOW    1
 
-class wxDataViewHeaderWindowBase : public wxControl
+// NB: for some reason, this class must be dllexport'ed or we get warnings from
+//     MSVC in DLL build
+class WXDLLIMPEXP_ADV wxDataViewHeaderWindowBase : public wxControl
 {
 public:
     wxDataViewHeaderWindowBase()