Add wxListCtrlBase::GetItemCount() pure virtual

No real changes, just ensure that all derived classes implement this
method (which already was the case).

include/wx/generic/listctrl.h

@@ -86,7 +86,7 @@ public:
     bool GetSubItemRect( long item, long subItem, wxRect& rect, int code = wxLIST_RECT_BOUNDS ) const;
     bool GetItemPosition( long item, wxPoint& pos ) const;
     bool SetItemPosition( long item, const wxPoint& pos ); // not supported in wxGLC
-    int GetItemCount() const;
+    int GetItemCount() const wxOVERRIDE;
     int GetColumnCount() const wxOVERRIDE;
     void SetItemSpacing( int spacing, bool isSmall = false );
     wxSize GetItemSpacing() const;