Compilation fix for STL build in wxGtkCollatableString code.

Include wx/string.h to define wxString used by this class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65357 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/private/string.h

@@ -36,6 +36,7 @@ private:
 // list for sorting collated strings
 // ----------------------------------------------------------------------------
 
+#include "wx/string.h"
 #include "wx/vector.h"
 #include "wx/sharedptr.h"