Avoid some GTK+ run-time errors when using wx{File,Dir}PickerCtrl

The wxGTK implementation uses wx{File,Dir}Button, which derives from
wxButton, but is not a GtkButton, so many operations which try to modify
it, such as changing it's style, are likely to result in errors.
See #17984

include/wx/gtk/filepicker.h

@@ -101,6 +101,8 @@ public:     // overrides
 protected:
     wxDialog *m_dialog;
 
+    virtual void DoApplyWidgetStyle(GtkRcStyle*) wxOVERRIDE;
+
 private:
     // common part of all ctors
     void Init() { m_dialog = NULL; }
@@ -170,6 +172,8 @@ public:     // overrides
 protected:
     wxDialog *m_dialog;
 
+    virtual void DoApplyWidgetStyle(GtkRcStyle*) wxOVERRIDE;
+
 public:    // used by the GTK callback only
 
     bool m_bIgnoreNextChange;