Declare AddTreeviewHeaderButton() only when it's defined

This method of wxGtkStyleContext uses a GTK function only available
since 3.20, and so can't be implemented for the earlier versions, hence
don't even define it in this case.

Note that the caller still needs to test for the run-time version.

include/wx/gtk/private/stylecontext.h

@@ -28,7 +28,9 @@ public:
     wxGtkStyleContext& AddTextview(const char* child1 = NULL, const char* child2 = NULL);
     wxGtkStyleContext& AddTooltip();
     wxGtkStyleContext& AddTreeview();
+#if GTK_CHECK_VERSION(3,20,0)
     wxGtkStyleContext& AddTreeviewHeaderButton(int pos);
+#endif // GTK >= 3.20
     wxGtkStyleContext& AddWindow(const char* className2 = NULL);
     void Bg(wxColour& color, int state = GTK_STATE_FLAG_NORMAL) const;
     void Fg(wxColour& color, int state = GTK_STATE_FLAG_NORMAL) const;