Fix setting wxBU_EXACTFIT style for wxButton under wxGTK2

The way wxBU_EXACTFIT is implemented in commit c1bb80987f
("Improve implementation of wxBU_EXACTFIT style for wxButton under wxGTK2",
2020-04-12) is too intrusive and disrupts button's appearance in some
themes. It should work better with themes if we reduce inner border in
a more GTK-compliant way by applying a dedicated GTK style to the button.

Closes #19081.

include/wx/gtk/button.h

@@ -67,6 +67,12 @@ private:
     // Return the GtkLabel used by this button.
     GtkLabel *GTKGetLabel() const;
 
+#ifndef __WXGTK3__
+    // To mark if special GTK style for buttons with wxBU_EXACTFIT flag
+    // was already defined.
+    static bool m_exactFitStyleDefined;
+#endif // !__WXGTK3__
+
     wxDECLARE_DYNAMIC_CLASS(wxButton);
 };