Fix _gtk_get_slowdown()

We need to pass void as the argument type.

gtk/gtkprivate.h

@@ -92,7 +92,7 @@ gboolean        _gtk_propagate_captured_event  (GtkWidget       *widget,
                                                 GtkWidget       *topmost);
 
 
-gdouble _gtk_get_slowdown ();
+gdouble _gtk_get_slowdown (void);
 void    _gtk_set_slowdown (gdouble slowdown_factor);
 
 gboolean gtk_should_use_portal (void);

gtk/gtkprogresstracker.c

@@ -45,7 +45,7 @@ _gtk_set_slowdown (gdouble factor)
 }
 
 gdouble
-_gtk_get_slowdown (gdouble factor)
+_gtk_get_slowdown (void)
 {
   return gtk_slowdown;
 }