Use DLL declarations consistently for wxCollapsibleHeaderCtrl

Export this class and its base classes (to avoid MSVC warnings about using
non-DLL-exported base classes for a DLL-exported one) from core DLL.

include/wx/generic/collheaderctrl.h

@@ -10,7 +10,8 @@
 #ifndef _WX_GENERIC_COLLAPSIBLEHEADER_CTRL_H_
 #define _WX_GENERIC_COLLAPSIBLEHEADER_CTRL_H_
 
-class wxGenericCollapsibleHeaderCtrl : public wxCollapsibleHeaderCtrlBase
+class WXDLLIMPEXP_CORE wxGenericCollapsibleHeaderCtrl
+    : public wxCollapsibleHeaderCtrlBase
 {
 public:
     wxGenericCollapsibleHeaderCtrl() { Init(); }